[lldb] Decouple importing the std C++ module from the way the program is compiled

Summary:
At the moment, when trying to import the `std` module in LLDB, we look at the imported modules used in the compiled program
and try to infer the Clang configuration we need from the DWARF module-import. That was the initial idea but turned out to
cause a few problems or inconveniences:

* It requires that users compile their programs with C++ modules. Given how experimental C++ modules are makes this feature inaccessible
for many users. Also it means that people can't just get the benefits of this feature for free when we activate it by default
(and we can't just close all the associated bug reports).
* Relying on DWARF's imported module tags (that are only emitted by default on macOS) means this can only be used when using DWARF (and with -glldb on Linux).
* We essentially hardcoded the C standard library paths on some platforms (Linux) or just couldn't support this feature on other platforms (macOS).

This patch drops the whole idea of looking at the imported module DWARF tags and instead just uses the support files of the compilation unit.
If we look at the support files and see file paths that indicate where the C standard library and libc++ are, we can just create the module
configuration this information. This fixes all the problems above which means we can enable all the tests now on Linux, macOS and with other debug information
than what we currently had. The only debug information specific code is now the iteration over external type module when -gmodules is used (as `std` and also the
`Darwin` module are their own external type module with their own files).

The meat of this patch is the CppModuleConfiguration which looks at the file paths from the compilation unit and then figures out the include paths
based on those paths. It's quite conservative in that it only enables modules if we find a single C library and single libc++ library. It's still missing some
test mode where we try to compile an expression before we actually activate the config for the user (which probably also needs some caching mechanism),
but for now it works and makes the feature usable.

Reviewers: aprantl, shafik, jdoerfert

Reviewed By: aprantl

Subscribers: mgorny, abidh, JDevlieghere, lldb-commits

Tags: #c_modules_in_lldb, #lldb

Differential Revision: https://reviews.llvm.org/D67760

llvm-svn: 372716

62 files changed, 437 insertions, 203 deletions

diff --git a/lldb/include/lldb/Symbol/CompileUnit.h b/lldb/include/lldb/Symbol/CompileUnit.h
index dc4896c9238..d132c367b99 100644
--- a/lldb/include/lldb/Symbol/CompileUnit.h
+++ b/lldb/include/lldb/Symbol/CompileUnit.h
@@ -225,6 +225,14 @@ public:
 
   DebugMacros *GetDebugMacros();
 
+  /// Apply a lambda to each external lldb::Module referenced by this
+  /// compilation unit. Recursively also descends into the referenced external
+  /// modules of any encountered compilation unit.
+  ///
+  /// \param[in] lambda
+  ///     The lambda that should be applied to every module.
+  void ForEachExternalModule(llvm::function_ref<void(lldb::ModuleSP)> f);
+
   /// Get the compile unit's support file list.
   ///
   /// The support file list is used by the line table, and any objects that
diff --git a/lldb/include/lldb/Symbol/SymbolFile.h b/lldb/include/lldb/Symbol/SymbolFile.h
index ce8da8014c5..d92b456f862 100644
--- a/lldb/include/lldb/Symbol/SymbolFile.h
+++ b/lldb/include/lldb/Symbol/SymbolFile.h
@@ -122,6 +122,9 @@ public:
   virtual size_t ParseFunctions(CompileUnit &comp_unit) = 0;
   virtual bool ParseLineTable(CompileUnit &comp_unit) = 0;
   virtual bool ParseDebugMacros(CompileUnit &comp_unit) = 0;
+  virtual void
+  ForEachExternalModule(CompileUnit &comp_unit,
+                        llvm::function_ref<void(lldb::ModuleSP)> f) {}
   virtual bool ParseSupportFiles(CompileUnit &comp_unit,
                                  FileSpecList &support_files) = 0;
   virtual size_t ParseTypes(CompileUnit &comp_unit) = 0;
diff --git a/lldb/include/lldb/Target/Platform.h b/lldb/include/lldb/Target/Platform.h
index 3ba58c0ec74..ba9e97be1b6 100644
--- a/lldb/include/lldb/Target/Platform.h
+++ b/lldb/include/lldb/Target/Platform.h
@@ -257,19 +257,6 @@ public:
 
   virtual bool SetRemoteWorkingDirectory(const FileSpec &working_dir);
 
-  /// Retrieve the system include directories on this platform for the
-  /// given language.
-  ///
-  /// \param[in] lang
-  ///     The language for which the include directories should be queried.
-  ///
-  /// \param[out] directories
-  ///     The include directories for this system.
-  virtual std::vector<std::string>
-  GetSystemIncludeDirectories(lldb::LanguageType lang) {
-    return {};
-  }
-
   virtual UserIDResolver &GetUserIDResolver() = 0;
 
   /// Locate a file for a platform.
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/TestImportStdModule.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/TestImportStdModule.py
index 19e02ca84c9..de0d796505e 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/TestImportStdModule.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/basic/TestImportStdModule.py
@@ -11,12 +11,8 @@ class ImportStdModule(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
@@ -34,12 +30,8 @@ class ImportStdModule(TestBase):
         self.expect("expr char char_a = 'b'; char char_b = 'a'; std::swap(char_a, char_b); char_a",
                     substrs=["(char) $3 = 'a'"])
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test_non_cpp_language(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/TestStdModuleWithConflicts.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/TestStdModuleWithConflicts.py
index 2abaece27fe..ad5322baab0 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/TestStdModuleWithConflicts.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/conflicts/TestStdModuleWithConflicts.py
@@ -16,12 +16,8 @@ class TestImportStdModuleConflicts(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/TestBasicDeque.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/TestBasicDeque.py
index 0ede19e8350..4615d249bb1 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/TestBasicDeque.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-basic/TestBasicDeque.py
@@ -10,12 +10,8 @@ class TestBasicDeque(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/TestDbgInfoContentDeque.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/TestDbgInfoContentDeque.py
index eeea34819dd..cde74af8e1f 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/TestDbgInfoContentDeque.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/deque-dbg-info-content/TestDbgInfoContentDeque.py
@@ -10,12 +10,8 @@ class TestDbgInfoContentDeque(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/TestBasicForwardList.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/TestBasicForwardList.py
index 528d555e0af..9e45bf23c12 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/TestBasicForwardList.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-basic/TestBasicForwardList.py
@@ -10,12 +10,8 @@ class TestBasicForwardList(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/TestDbgInfoContentForwardList.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/TestDbgInfoContentForwardList.py
index a2fc5598930..290a0450fc8 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/TestDbgInfoContentForwardList.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/forward_list-dbg-info-content/TestDbgInfoContentForwardList.py
@@ -10,12 +10,8 @@ class TestDbgInfoContentForwardList(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/TestBasicList.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/TestBasicList.py
index 154c8ed7bd0..e8dfc884739 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/TestBasicList.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-basic/TestBasicList.py
@@ -10,12 +10,8 @@ class TestBasicList(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/TestDbgInfoContentList.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/TestDbgInfoContentList.py
index 713ec1abb85..fa8be7cabee 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/TestDbgInfoContentList.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/list-dbg-info-content/TestDbgInfoContentList.py
@@ -11,12 +11,8 @@ class TestDbgInfoContentList(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/TestMissingStdModule.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/TestMissingStdModule.py
index f4d3d1fd492..83d672fb1fd 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/TestMissingStdModule.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/no-std-module/TestMissingStdModule.py
@@ -17,12 +17,8 @@ class STLTestCase(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/TestQueue.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/TestQueue.py
index aad1b11719b..42e5f8d8c43 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/TestQueue.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/queue/TestQueue.py
@@ -10,12 +10,8 @@ class TestQueue(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/TestSharedPtrDbgInfoContent.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/TestSharedPtrDbgInfoContent.py
index 028798f72f3..ec39651b815 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/TestSharedPtrDbgInfoContent.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr-dbg-info-content/TestSharedPtrDbgInfoContent.py
@@ -10,12 +10,8 @@ class TestSharedPtrDbgInfoContent(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/TestSharedPtr.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/TestSharedPtr.py
index 6f1e7b12d1f..7e05bcca477 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/TestSharedPtr.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/shared_ptr/TestSharedPtr.py
@@ -10,12 +10,8 @@ class TestSharedPtr(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/TestStack.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/TestStack.py
index 1c2e5da54ed..fe19a4e5f1b 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/TestStack.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/stack/TestStack.py
@@ -10,12 +10,8 @@ class TestStack(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/Makefile
index 459a66c5c8b..4aa2b432f12 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/Makefile
@@ -3,7 +3,6 @@
 # system headers.
 NO_TEST_COMMON_H := 1
 
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXXFLAGS += -I $(SRCDIR)/root/usr/include/c++/include/ -I $(SRCDIR)/root/usr/include/ -nostdinc -nostdinc++ -nostdlib++
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/TestStdModuleSysroot.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/TestStdModuleSysroot.py
index dfd90e80e45..fbf00cddcfa 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/TestStdModuleSysroot.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/sysroot/TestStdModuleSysroot.py
@@ -11,11 +11,7 @@ class ImportStdModule(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/TestUniquePtrDbgInfoContent.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/TestUniquePtrDbgInfoContent.py
index c321d8022e1..9389de97128 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/TestUniquePtrDbgInfoContent.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr-dbg-info-content/TestUniquePtrDbgInfoContent.py
@@ -10,12 +10,8 @@ class TestUniquePtrDbgInfoContent(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/TestUniquePtr.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/TestUniquePtr.py
index 295ec512327..a0e05b2c420 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/TestUniquePtr.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/unique_ptr/TestUniquePtr.py
@@ -10,12 +10,8 @@ class TestUniquePtr(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/TestBasicVector.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/TestBasicVector.py
index deaf8f0cb75..840bacb86b3 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/TestBasicVector.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-basic/TestBasicVector.py
@@ -10,12 +10,8 @@ class TestBasicVector(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/TestBoolVector.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/TestBoolVector.py
index 0ab52695faf..45a2507f776 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/TestBoolVector.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-bool/TestBoolVector.py
@@ -10,12 +10,8 @@ class TestBoolVector(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/TestDbgInfoContentVector.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/TestDbgInfoContentVector.py
index c89265f2cd3..4d51f1ba3b6 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/TestDbgInfoContentVector.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-dbg-info-content/TestDbgInfoContentVector.py
@@ -11,12 +11,8 @@ class TestDbgInfoContentVector(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/TestVectorOfVectors.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/TestVectorOfVectors.py
index 2b82e1a039a..feaeac9be5e 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/TestVectorOfVectors.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/vector-of-vectors/TestVectorOfVectors.py
@@ -10,12 +10,8 @@ class TestVectorOfVectors(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/TestDbgInfoContentWeakPtr.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/TestDbgInfoContentWeakPtr.py
index 0acd54e803c..5ed30312ea7 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/TestDbgInfoContentWeakPtr.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr-dbg-info-content/TestDbgInfoContentWeakPtr.py
@@ -10,12 +10,8 @@ class TestDbgInfoContentWeakPtr(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/Makefile b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/Makefile
index 12e489d83b2..f938f742846 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/Makefile
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/Makefile
@@ -1,4 +1,3 @@
 USE_LIBCPP := 1
-CXXFLAGS += $(MANDATORY_CXXMODULE_BUILD_CFLAGS)
 CXX_SOURCES := main.cpp
 include Makefile.rules
diff --git a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/TestWeakPtr.py b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/TestWeakPtr.py
index 6b8b9ceb7aa..76241740b68 100644
--- a/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/TestWeakPtr.py
+++ b/lldb/packages/Python/lldbsuite/test/commands/expression/import-std-module/weak_ptr/TestWeakPtr.py
@@ -10,12 +10,8 @@ class TestSharedPtr(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    # FIXME: This should work on more setups, so remove these
-    # skipIf's in the future.
     @add_test_categories(["libc++"])
     @skipIf(compiler=no_match("clang"))
-    @skipIf(oslist=no_match(["linux"]))
-    @skipIf(debug_info=no_match(["dwarf"]))
     def test(self):
         self.build()
 
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/CMakeLists.txt b/lldb/source/Plugins/ExpressionParser/Clang/CMakeLists.txt
index 99d50c4007f..b114ce7762d 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/CMakeLists.txt
+++ b/lldb/source/Plugins/ExpressionParser/Clang/CMakeLists.txt
@@ -19,6 +19,7 @@ add_lldb_library(lldbPluginExpressionParserClang PLUGIN
   ClangPersistentVariables.cpp
   ClangUserExpression.cpp
   ClangUtilityFunction.cpp
+  CppModuleConfiguration.cpp
   IRForTarget.cpp
   IRDynamicChecks.cpp
 
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
index cb0e985b8cf..593348fbeb0 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
@@ -241,38 +241,23 @@ static void SetupModuleHeaderPaths(CompilerInstance *compiler,
   search_opts.ModuleCachePath = module_cache.str();
   LLDB_LOG(log, "Using module cache path: {0}", module_cache.c_str());
 
-  FileSpec clang_resource_dir = GetClangResourceDir();
-  std::string resource_dir = clang_resource_dir.GetPath();
-  if (FileSystem::Instance().IsDirectory(resource_dir)) {
-    search_opts.ResourceDir = resource_dir;
-    std::string resource_include = resource_dir + "/include";
-    search_opts.AddPath(resource_include, frontend::System, false, true);
-
-    LLDB_LOG(log, "Added resource include dir: {0}", resource_include);
-  }
+  search_opts.ResourceDir = GetClangResourceDir().GetPath();
 
   search_opts.ImplicitModuleMaps = true;
-
-  std::vector<std::string> system_include_directories =
-      target_sp->GetPlatform()->GetSystemIncludeDirectories(
-          lldb::eLanguageTypeC_plus_plus);
-
-  for (const std::string &include_dir : system_include_directories) {
-    search_opts.AddPath(include_dir, frontend::System, false, true);
-
-    LLDB_LOG(log, "Added system include dir: {0}", include_dir);
-  }
 }
 
 //===----------------------------------------------------------------------===//
 // Implementation of ClangExpressionParser
 //===----------------------------------------------------------------------===//
 
-ClangExpressionParser::ClangExpressionParser(ExecutionContextScope *exe_scope, Expression &expr,
-    bool generate_debug_info, std::vector<std::string> include_directories, std::string filename)
+ClangExpressionParser::ClangExpressionParser(
+    ExecutionContextScope *exe_scope, Expression &expr,
+    bool generate_debug_info, std::vector<std::string> include_directories,
+    std::string filename)
     : ExpressionParser(exe_scope, expr, generate_debug_info), m_compiler(),
       m_pp_callbacks(nullptr),
-      m_include_directories(std::move(include_directories)), m_filename(std::move(filename)) {
+      m_include_directories(std::move(include_directories)),
+      m_filename(std::move(filename)) {
   Log *log(lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_EXPRESSIONS));
 
   // We can't compile expressions without a target.  So if the exe_scope is
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
index ae95c687981..813d03c7194 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
@@ -25,6 +25,7 @@
 #include "ClangExpressionParser.h"
 #include "ClangModulesDeclVendor.h"
 #include "ClangPersistentVariables.h"
+#include "CppModuleConfiguration.h"
 
 #include "lldb/Core/Debugger.h"
 #include "lldb/Core/Module.h"
@@ -433,48 +434,59 @@ static bool SupportsCxxModuleImport(lldb::LanguageType language) {
   }
 }
 
-std::vector<std::string>
-ClangUserExpression::GetModulesToImport(ExecutionContext &exe_ctx) {
+/// Utility method that puts a message into the expression log and
+/// returns an invalid module configuration.
+static CppModuleConfiguration LogConfigError(const std::string &msg) {
   Log *log(lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_EXPRESSIONS));
+  LLDB_LOG(log, "[C++ module config] {0}", msg);
+  return CppModuleConfiguration();
+}
 
-  if (!SupportsCxxModuleImport(Language()))
-    return {};
+CppModuleConfiguration GetModuleConfig(lldb::LanguageType language,
+                                       ExecutionContext &exe_ctx) {
+  // Don't do anything if this is not a C++ module configuration.
+  if (!SupportsCxxModuleImport(language))
+    return LogConfigError("Language doesn't support C++ modules");
 
   Target *target = exe_ctx.GetTargetPtr();
-  if (!target || !target->GetEnableImportStdModule())
-    return {};
+  if (!target)
+    return LogConfigError("No target");
+
+  if (!target->GetEnableImportStdModule())
+    return LogConfigError("Importing std module not enabled in settings");
 
   StackFrame *frame = exe_ctx.GetFramePtr();
   if (!frame)
-    return {};
+    return LogConfigError("No frame");
 
   Block *block = frame->GetFrameBlock();
   if (!block)
-    return {};
+    return LogConfigError("No block");
 
   SymbolContext sc;
   block->CalculateSymbolContext(&sc);
   if (!sc.comp_unit)
-    return {};
-
-  if (log) {
-    for (const SourceModule &m : sc.comp_unit->GetImportedModules()) {
-      LLDB_LOG(log, "Found module in compile unit: {0:$[.]} - include dir: {1}",
-                  llvm::make_range(m.path.begin(), m.path.end()), m.search_path);
+    return LogConfigError("Couldn't calculate symbol context");
+
+  // Build a list of files we need to analyze to build the configuration.
+  FileSpecList files;
+  for (const FileSpec &f : sc.comp_unit->GetSupportFiles())
+    files.AppendIfUnique(f);
+  // We also need to look at external modules in the case of -gmodules as they
+  // contain the support files for libc++ and the C library.
+  sc.comp_unit->ForEachExternalModule([&files](lldb::ModuleSP module) {
+    for (std::size_t i = 0; i < module->GetNumCompileUnits(); ++i) {
+      const FileSpecList &support_files =
+          module->GetCompileUnitAtIndex(i)->GetSupportFiles();
+      for (const FileSpec &f : support_files) {
+        files.AppendIfUnique(f);
+      }
     }
-  }
-
-  for (const SourceModule &m : sc.comp_unit->GetImportedModules())
-    m_include_directories.emplace_back(m.search_path.GetCString());
-
-  // Check if we imported 'std' or any of its submodules.
-  // We currently don't support importing any other modules in the expression
-  // parser.
-  for (const SourceModule &m : sc.comp_unit->GetImportedModules())
-    if (!m.path.empty() && m.path.front() == "std")
-      return {"std"};
-
-  return {};
+  });
+  // Try to create a configuration from the files. If there is no valid
+  // configuration possible with the files, this just returns an invalid
+  // configuration.
+  return CppModuleConfiguration(files);
 }
 
 bool ClangUserExpression::PrepareForParsing(
@@ -502,14 +514,21 @@ bool ClangUserExpression::PrepareForParsing(
 
   SetupDeclVendor(exe_ctx, m_target);
 
-  std::vector<std::string> used_modules = GetModulesToImport(exe_ctx);
-  m_imported_cpp_modules = !used_modules.empty();
+  CppModuleConfiguration module_config = GetModuleConfig(m_language, exe_ctx);
+  llvm::ArrayRef<std::string> imported_modules =
+      module_config.GetImportedModules();
+  m_imported_cpp_modules = !imported_modules.empty();
+  m_include_directories = module_config.GetIncludeDirs();
 
   LLDB_LOG(log, "List of imported modules in expression: {0}",
-           llvm::make_range(used_modules.begin(), used_modules.end()));
+           llvm::make_range(imported_modules.begin(), imported_modules.end()));
+  LLDB_LOG(log, "List of include directories gathered for modules: {0}",
+           llvm::make_range(m_include_directories.begin(),
+                            m_include_directories.end()));
 
   UpdateLanguageForExpr();
-  CreateSourceCode(diagnostic_manager, exe_ctx, used_modules, for_completion);
+  CreateSourceCode(diagnostic_manager, exe_ctx, imported_modules,
+                   for_completion);
   return true;
 }
 
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.h b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.h
index ac28ac4a372..d94f9cc5e06 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.h
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.h
@@ -179,7 +179,6 @@ private:
                     lldb::addr_t struct_address,
                     DiagnosticManager &diagnostic_manager) override;
 
-  std::vector<std::string> GetModulesToImport(ExecutionContext &exe_ctx);
   void CreateSourceCode(DiagnosticManager &diagnostic_manager,
                         ExecutionContext &exe_ctx,
                         std::vector<std::string> modules_to_import,
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.cpp b/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.cpp
new file mode 100644
index 00000000000..47c0345d6a0
--- /dev/null
+++ b/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.cpp
@@ -0,0 +1,82 @@
+//===-- CppModuleConfiguration.cpp ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CppModuleConfiguration.h"
+
+#include "ClangHost.h"
+#include "lldb/Host/FileSystem.h"
+
+using namespace lldb_private;
+
+bool CppModuleConfiguration::SetOncePath::TrySet(llvm::StringRef path) {
+  // Setting for the first time always works.
+  if (m_first) {
+    m_path = path.str();
+    m_valid = true;
+    m_first = false;
+    return true;
+  }
+  // Changing the path to the same value is fine.
+  if (m_path == path)
+    return true;
+
+  // Changing the path after it was already set is not allowed.
+  m_valid = false;
+  return false;
+}
+
+bool CppModuleConfiguration::analyzeFile(const FileSpec &f) {
+  llvm::SmallString<256> dir_buffer = f.GetDirectory().GetStringRef();
+  // Convert to posix style so that we can use '/'.
+  llvm::sys::path::native(dir_buffer, llvm::sys::path::Style::posix);
+  llvm::StringRef posix_dir(dir_buffer);
+
+  // Check for /c++/vX/ that is used by libc++.
+  static llvm::Regex libcpp_regex(R"regex(/c[+][+]/v[0-9]/)regex");
+  if (libcpp_regex.match(f.GetPath())) {
+    // Strip away libc++'s /experimental directory if there is one.
+    posix_dir.consume_back("/experimental");
+    return m_std_inc.TrySet(posix_dir);
+  }
+
+  // Check for /usr/include. On Linux this might be /usr/include/bits, so
+  // we should remove that '/bits' suffix to get the actual include directory.
+  if (posix_dir.endswith("/usr/include/bits"))
+    posix_dir.consume_back("/bits");
+  if (posix_dir.endswith("/usr/include"))
+    return m_c_inc.TrySet(posix_dir);
+
+  // File wasn't interesting, continue analyzing.
+  return true;
+}
+
+bool CppModuleConfiguration::hasValidConfig() {
+  // We all these include directories to have a valid usable configuration.
+  return m_c_inc.Valid() && m_std_inc.Valid();
+}
+
+CppModuleConfiguration::CppModuleConfiguration(
+    const FileSpecList &support_files) {
+  // Analyze all files we were given to build the configuration.
+  bool error = !std::all_of(support_files.begin(), support_files.end(),
+                            std::bind(&CppModuleConfiguration::analyzeFile,
+                                      this, std::placeholders::_1));
+  // If we have a valid configuration at this point, set the
+  // include directories and module list that should be used.
+  if (!error && hasValidConfig()) {
+    // Calculate the resource directory for LLDB.
+    llvm::SmallString<256> resource_dir;
+    llvm::sys::path::append(resource_dir, GetClangResourceDir().GetPath(),
+                            "include");
+    m_resource_inc = resource_dir.str();
+
+    // This order matches the way Clang orders these directories.
+    m_include_dirs = {m_std_inc.Get(), m_resource_inc, m_c_inc.Get()};
+    m_imported_modules = {"std"};
+  }
+}
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.h b/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.h
new file mode 100644
index 00000000000..8e892e37d0d
--- /dev/null
+++ b/lldb/source/Plugins/ExpressionParser/Clang/CppModuleConfiguration.h
@@ -0,0 +1,84 @@
+//===-- CppModuleConfiguration.h --------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef liblldb_CppModuleConfiguration_h_
+#define liblldb_CppModuleConfiguration_h_
+
+#include <lldb/Core/FileSpecList.h>
+#include <llvm/Support/Regex.h>
+
+namespace lldb_private {
+
+/// A Clang configuration when importing C++ modules.
+///
+/// Includes a list of include paths that should be used when importing
+/// and a list of modules that can be imported. Currently only used when
+/// importing the 'std' module and its dependencies.
+class CppModuleConfiguration {
+  /// Utility class for a path that can only be set once.
+  class SetOncePath {
+    std::string m_path;
+    bool m_valid = false;
+    /// True iff this path hasn't been set yet.
+    bool m_first = true;
+
+  public:
+    /// Try setting the path. Returns true if the path was set and false if
+    /// the path was already set.
+    LLVM_NODISCARD bool TrySet(llvm::StringRef path);
+    /// Return the path if there is one.
+    std::string Get() const {
+      assert(m_valid && "Called Get() on an invalid SetOncePath?");
+      return m_path;
+    }
+    /// Returns true iff this path was set exactly once so far.
+    bool Valid() const { return m_valid; }
+  };
+
+  /// If valid, the include path used for the std module.
+  SetOncePath m_std_inc;
+  /// If valid, the include path to the C library (e.g. /usr/include).
+  SetOncePath m_c_inc;
+  /// The Clang resource include path for this configuration.
+  std::string m_resource_inc;
+
+  std::vector<std::string> m_include_dirs;
+  std::vector<std::string> m_imported_modules;
+
+  /// Analyze a given source file to build the current configuration.
+  /// Returns false iff there was a fatal error that makes analyzing any
+  /// further files pointless as the configuration is now invalid.
+  bool analyzeFile(const FileSpec &f);
+
+public:
+  /// Creates a configuraiton by analyzing the given list of used source files.
+  ///
+  /// Currently only looks at the used paths and doesn't actually access the
+  /// files on the disk.
+  explicit CppModuleConfiguration(const FileSpecList &support_files);
+  /// Creates an empty and invalid configuration.
+  CppModuleConfiguration() {}
+
+  /// Returns true iff this is a valid configuration that can be used to
+  /// load and compile modules.
+  bool hasValidConfig();
+
+  /// Returns a list of include directories that should be used when using this
+  /// configuration (e.g. {"/usr/include", "/usr/include/c++/v1"}).
+  llvm::ArrayRef<std::string> GetIncludeDirs() const { return m_include_dirs; }
+
+  /// Returns a list of (top level) modules that should be imported when using
+  /// this configuration (e.g. {"std"}).
+  llvm::ArrayRef<std::string> GetImportedModules() const {
+    return m_imported_modules;
+  }
+};
+
+} // namespace lldb_private
+
+#endif
diff --git a/lldb/source/Plugins/Platform/Linux/PlatformLinux.cpp b/lldb/source/Plugins/Platform/Linux/PlatformLinux.cpp
index 52d72f5e6fd..f17fb8f0483 100644
--- a/lldb/source/Plugins/Platform/Linux/PlatformLinux.cpp
+++ b/lldb/source/Plugins/Platform/Linux/PlatformLinux.cpp
@@ -260,25 +260,6 @@ bool PlatformLinux::CanDebugProcess() {
   }
 }
 
-std::vector<std::string>
-PlatformLinux::GetSystemIncludeDirectories(lldb::LanguageType lang) {
-  std::string sys_root = GetSDKRootDirectory().AsCString("");
-  switch (lang) {
-  case lldb::eLanguageTypeC:
-  case lldb::eLanguageTypeC89:
-  case lldb::eLanguageTypeC99:
-  case lldb::eLanguageTypeC11:
-  case lldb::eLanguageTypeC_plus_plus:
-  case lldb::eLanguageTypeC_plus_plus_03:
-  case lldb::eLanguageTypeC_plus_plus_11:
-  case lldb::eLanguageTypeC_plus_plus_14:
-  case lldb::eLanguageTypeObjC_plus_plus:
-    return {sys_root + "/usr/include/"};
-  default:
-    return {};
-  }
-}
-
 // For local debugging, Linux will override the debug logic to use llgs-launch
 // rather than lldb-launch, llgs-attach.  This differs from current lldb-
 // launch, debugserver-attach approach on MacOSX.
diff --git a/lldb/source/Plugins/Platform/Linux/PlatformLinux.h b/lldb/source/Plugins/Platform/Linux/PlatformLinux.h
index a843558f1df..67d211ac824 100644
--- a/lldb/source/Plugins/Platform/Linux/PlatformLinux.h
+++ b/lldb/source/Plugins/Platform/Linux/PlatformLinux.h
@@ -48,9 +48,6 @@ public:
 
   bool CanDebugProcess() override;
 
-  std::vector<std::string>
-  GetSystemIncludeDirectories(lldb::LanguageType lang) override;
-
   lldb::ProcessSP DebugProcess(ProcessLaunchInfo &launch_info,
                                Debugger &debugger, Target *target,
                                Status &error) override;
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
index 08b9434f68e..2177cbc8957 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
@@ -845,6 +845,18 @@ size_t SymbolFileDWARF::ParseFunctions(CompileUnit &comp_unit) {
   return functions_added;
 }
 
+void SymbolFileDWARF::ForEachExternalModule(
+    CompileUnit &comp_unit, llvm::function_ref<void(ModuleSP)> f) {
+  UpdateExternalModuleListIfNeeded();
+
+  for (auto &p : m_external_type_modules) {
+    ModuleSP module = p.second;
+    f(module);
+    for (std::size_t i = 0; i < module->GetNumCompileUnits(); ++i)
+      module->GetCompileUnitAtIndex(i)->ForEachExternalModule(f);
+  }
+}
+
 bool SymbolFileDWARF::ParseSupportFiles(CompileUnit &comp_unit,
                                         FileSpecList &support_files) {
   if (!comp_unit.GetLineTable())
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h
index 2a1eb8ab9ab..6825c7b085b 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h
@@ -105,6 +105,10 @@ public:
 
   bool ParseDebugMacros(lldb_private::CompileUnit &comp_unit) override;
 
+  void
+  ForEachExternalModule(lldb_private::CompileUnit &comp_unit,
+                        llvm::function_ref<void(lldb::ModuleSP)> f) override;
+
   bool ParseSupportFiles(lldb_private::CompileUnit &comp_unit,
                          lldb_private::FileSpecList &support_files) override;
 
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp
index 77e2fcf887c..b079855ccbd 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp
@@ -652,6 +652,14 @@ bool SymbolFileDWARFDebugMap::ParseDebugMacros(CompileUnit &comp_unit) {
   return false;
 }
 
+void SymbolFileDWARFDebugMap::ForEachExternalModule(
+    CompileUnit &comp_unit, llvm::function_ref<void(ModuleSP)> f) {
+  std::lock_guard<std::recursive_mutex> guard(GetModuleMutex());
+  SymbolFileDWARF *oso_dwarf = GetSymbolFile(comp_unit);
+  if (oso_dwarf)
+    oso_dwarf->ForEachExternalModule(comp_unit, f);
+}
+
 bool SymbolFileDWARFDebugMap::ParseSupportFiles(CompileUnit &comp_unit,
                                                 FileSpecList &support_files) {
   std::lock_guard<std::recursive_mutex> guard(GetModuleMutex());
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
index 0b4f96b2036..2cc830485b2 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
@@ -53,6 +53,10 @@ public:
 
   bool ParseDebugMacros(lldb_private::CompileUnit &comp_unit) override;
 
+  void
+  ForEachExternalModule(lldb_private::CompileUnit &comp_unit,
+                        llvm::function_ref<void(lldb::ModuleSP)> f) override;
+
   bool ParseSupportFiles(lldb_private::CompileUnit &comp_unit,
                          lldb_private::FileSpecList &support_files) override;
 
diff --git a/lldb/source/Symbol/CompileUnit.cpp b/lldb/source/Symbol/CompileUnit.cpp
index dc50e832aa6..41086d2df3d 100644
--- a/lldb/source/Symbol/CompileUnit.cpp
+++ b/lldb/source/Symbol/CompileUnit.cpp
@@ -353,6 +353,11 @@ const std::vector<SourceModule> &CompileUnit::GetImportedModules() {
   return m_imported_modules;
 }
 
+void CompileUnit::ForEachExternalModule(llvm::function_ref<void(ModuleSP)> f) {
+  if (SymbolFile *symfile = GetModule()->GetSymbolFile())
+    symfile->ForEachExternalModule(*this, f);
+}
+
 const FileSpecList &CompileUnit::GetSupportFiles() {
   if (m_support_files.GetSize() == 0) {
     if (m_flags.IsClear(flagsParsedSupportFiles)) {
diff --git a/lldb/unittests/Expression/CMakeLists.txt b/lldb/unittests/Expression/CMakeLists.txt
index 2dcc8d3d655..3408b278b0b 100644
--- a/lldb/unittests/Expression/CMakeLists.txt
+++ b/lldb/unittests/Expression/CMakeLists.txt
@@ -2,6 +2,7 @@ add_lldb_unittest(ExpressionTests
   ClangParserTest.cpp
   DiagnosticManagerTest.cpp
   DWARFExpressionTest.cpp
+  CppModuleConfigurationTest.cpp
 
   LINK_LIBS
     lldbCore
diff --git a/lldb/unittests/Expression/CppModuleConfigurationTest.cpp b/lldb/unittests/Expression/CppModuleConfigurationTest.cpp
new file mode 100644
index 00000000000..135ce478636
--- /dev/null
+++ b/lldb/unittests/Expression/CppModuleConfigurationTest.cpp
@@ -0,0 +1,168 @@
+//===-- CppModuleConfigurationTest.cpp ---------------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/ExpressionParser/Clang/CppModuleConfiguration.h"
+#include "Plugins/ExpressionParser/Clang/ClangHost.h"
+#include "lldb/Host/FileSystem.h"
+#include "lldb/Host/HostInfo.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+using namespace lldb_private;
+
+namespace {
+struct CppModuleConfigurationTest : public testing::Test {
+  static void SetUpTestCase() {
+    // Getting the resource directory uses those subsystems, so we should
+    // initialize them.
+    FileSystem::Initialize();
+    HostInfo::Initialize();
+  }
+  static void TearDownTestCase() {
+    HostInfo::Terminate();
+    FileSystem::Terminate();
+  }
+};
+} // namespace
+
+/// Returns the Clang resource include directory.
+static std::string ResourceInc() {
+  llvm::SmallString<256> resource_dir;
+  llvm::sys::path::append(resource_dir, GetClangResourceDir().GetPath(),
+                          "include");
+  return resource_dir.str().str();
+}
+
+/// Utility function turningn a list of paths into a FileSpecList.
+static FileSpecList makeFiles(llvm::ArrayRef<std::string> paths) {
+  FileSpecList result;
+  for (const std::string &path : paths)
+    result.Append(FileSpec(path));
+  return result;
+}
+
+TEST_F(CppModuleConfigurationTest, Linux) {
+  // Test the average Linux configuration.
+  std::string libcpp = "/usr/include/c++/v1";
+  std::string usr = "/usr/include";
+  CppModuleConfiguration config(
+      makeFiles({usr + "/bits/types.h", libcpp + "/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre(libcpp, ResourceInc(), usr));
+}
+
+TEST_F(CppModuleConfigurationTest, Sysroot) {
+  // Test that having a sysroot for the whole system works fine.
+  std::string libcpp = "/home/user/sysroot/usr/include/c++/v1";
+  std::string usr = "/home/user/sysroot/usr/include";
+  CppModuleConfiguration config(
+      makeFiles({usr + "/bits/types.h", libcpp + "/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre(libcpp, ResourceInc(), usr));
+}
+
+TEST_F(CppModuleConfigurationTest, LinuxLocalLibCpp) {
+  // Test that a locally build libc++ is detected.
+  std::string libcpp = "/home/user/llvm-build/include/c++/v1";
+  std::string usr = "/usr/include";
+  CppModuleConfiguration config(
+      makeFiles({usr + "/bits/types.h", libcpp + "/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre(libcpp, ResourceInc(), usr));
+}
+
+TEST_F(CppModuleConfigurationTest, UnrelatedLibrary) {
+  // Test that having an unrelated library in /usr/include doesn't break.
+  std::string libcpp = "/home/user/llvm-build/include/c++/v1";
+  std::string usr = "/usr/include";
+  CppModuleConfiguration config(makeFiles(
+      {usr + "/bits/types.h", libcpp + "/vector", usr + "/boost/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre(libcpp, ResourceInc(), usr));
+}
+
+TEST_F(CppModuleConfigurationTest, Xcode) {
+  // Test detection of libc++ coming from Xcode with generic platform names.
+  std::string p = "/Applications/Xcode.app/Contents/Developer/";
+  std::string libcpp = p + "Toolchains/B.xctoolchain/usr/include/c++/v1";
+  std::string usr =
+      p + "Platforms/A.platform/Developer/SDKs/OSVers.sdk/usr/include";
+  CppModuleConfiguration config(
+      makeFiles({libcpp + "/unordered_map", usr + "/stdio.h"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre(libcpp, ResourceInc(), usr));
+}
+
+TEST_F(CppModuleConfigurationTest, LibCppV2) {
+  // Test that a "v2" of libc++ is still correctly detected.
+  CppModuleConfiguration config(
+      makeFiles({"/usr/include/bits/types.h", "/usr/include/c++/v2/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre("/usr/include/c++/v2", ResourceInc(),
+                                   "/usr/include"));
+}
+
+TEST_F(CppModuleConfigurationTest, UnknownLibCppFile) {
+  // Test that having some unknown file in the libc++ path doesn't break
+  // anything.
+  CppModuleConfiguration config(makeFiles(
+      {"/usr/include/bits/types.h", "/usr/include/c++/v1/non_existing_file"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre("std"));
+  EXPECT_THAT(config.GetIncludeDirs(),
+              testing::ElementsAre("/usr/include/c++/v1", ResourceInc(),
+                                   "/usr/include"));
+}
+
+TEST_F(CppModuleConfigurationTest, MissingUsrInclude) {
+  // Test that we don't load 'std' if we can't find the C standard library.
+  CppModuleConfiguration config(makeFiles({"/usr/include/c++/v1/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre());
+  EXPECT_THAT(config.GetIncludeDirs(), testing::ElementsAre());
+}
+
+TEST_F(CppModuleConfigurationTest, MissingLibCpp) {
+  // Test that we don't load 'std' if we don't have a libc++.
+  CppModuleConfiguration config(makeFiles({"/usr/include/bits/types.h"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre());
+  EXPECT_THAT(config.GetIncludeDirs(), testing::ElementsAre());
+}
+
+TEST_F(CppModuleConfigurationTest, IgnoreLibStdCpp) {
+  // Test that we don't do anything bad when we encounter libstdc++ paths.
+  CppModuleConfiguration config(makeFiles(
+      {"/usr/include/bits/types.h", "/usr/include/c++/8.0.1/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre());
+  EXPECT_THAT(config.GetIncludeDirs(), testing::ElementsAre());
+}
+
+TEST_F(CppModuleConfigurationTest, AmbiguousCLib) {
+  // Test that we don't do anything when we are not sure where the
+  // right C standard library is.
+  CppModuleConfiguration config(
+      makeFiles({"/usr/include/bits/types.h", "/usr/include/c++/v1/vector",
+                 "/sysroot/usr/include/bits/types.h"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre());
+  EXPECT_THAT(config.GetIncludeDirs(), testing::ElementsAre());
+}
+
+TEST_F(CppModuleConfigurationTest, AmbiguousLibCpp) {
+  // Test that we don't do anything when we are not sure where the
+  // right libc++ is.
+  CppModuleConfiguration config(
+      makeFiles({"/usr/include/bits/types.h", "/usr/include/c++/v1/vector",
+                 "/usr/include/c++/v2/vector"}));
+  EXPECT_THAT(config.GetImportedModules(), testing::ElementsAre());
+  EXPECT_THAT(config.GetIncludeDirs(), testing::ElementsAre());
+}