3 files changed, 47 insertions, 7 deletions

diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index fa2a76ef47c..c667f4bf220 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1092,13 +1092,51 @@ void HeaderSearch::MarkFileModuleHeader(const FileEntry *FE,
 }
 
 bool HeaderSearch::ShouldEnterIncludeFile(Preprocessor &PP,
-                                          const FileEntry *File,
-                                          bool isImport, Module *M) {
+                                          const FileEntry *File, bool isImport,
+                                          bool ModulesEnabled, Module *M) {
   ++NumIncluded; // Count # of attempted #includes.
 
   // Get information about this file.
   HeaderFileInfo &FileInfo = getFileInfo(File);
 
+  // FIXME: this is a workaround for the lack of proper modules-aware support
+  // for #import / #pragma once
+  auto TryEnterImported = [&](void) -> bool {
+    if (!ModulesEnabled)
+      return false;
+    // Modules with builtins are special; multiple modules use builtins as
+    // modular headers, example:
+    //
+    //    module stddef { header "stddef.h" export * }
+    //
+    // After module map parsing, this expands to:
+    //
+    //    module stddef {
+    //      header "/path_to_builtin_dirs/stddef.h"
+    //      textual "stddef.h"
+    //    }
+    //
+    // It's common that libc++ and system modules will both define such
+    // submodules. Make sure cached results for a builtin header won't
+    // prevent other builtin modules to potentially enter the builtin header.
+    // Note that builtins are header guarded and the decision to actually
+    // enter them is postponed to the controlling macros logic below.
+    bool TryEnterHdr = false;
+    if (FileInfo.isCompilingModuleHeader && FileInfo.isModuleHeader)
+      TryEnterHdr = File->getDir() == ModMap.getBuiltinDir() &&
+                    ModuleMap::isBuiltinHeader(
+                        llvm::sys::path::filename(File->getName()));
+
+    // Textual headers can be #imported from different modules. Since ObjC
+    // headers find in the wild might rely only on #import and do not contain
+    // controlling macros, be conservative and only try to enter textual headers
+    // if such macro is present.
+    if (!FileInfo.isModuleHeader &&
+        FileInfo.getControllingMacro(ExternalLookup))
+      TryEnterHdr = true;
+    return TryEnterHdr;
+  };
+
   // If this is a #import directive, check that we have not already imported
   // this header.
   if (isImport) {
@@ -1106,11 +1144,12 @@ bool HeaderSearch::ShouldEnterIncludeFile(Preprocessor &PP,
     FileInfo.isImport = true;
 
     // Has this already been #import'ed or #include'd?
-    if (FileInfo.NumIncludes) return false;
+    if (FileInfo.NumIncludes && !TryEnterImported())
+      return false;
   } else {
     // Otherwise, if this is a #include of a file that was previously #import'd
     // or if this is the second #include of a #pragma once file, ignore it.
-    if (FileInfo.isImport)
+    if (FileInfo.isImport && !TryEnterImported())
       return false;
   }
 
diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index 9d0f2eb2fa7..db834ede022 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -144,7 +144,7 @@ static StringRef sanitizeFilenameAsIdentifier(StringRef Name,
 /// \brief Determine whether the given file name is the name of a builtin
 /// header, supplied by Clang to replace, override, or augment existing system
 /// headers.
-static bool isBuiltinHeader(StringRef FileName) {
+bool ModuleMap::isBuiltinHeader(StringRef FileName) {
   return llvm::StringSwitch<bool>(FileName)
            .Case("float.h", true)
            .Case("iso646.h", true)
@@ -165,7 +165,7 @@ ModuleMap::findKnownHeader(const FileEntry *File) {
   HeadersMap::iterator Known = Headers.find(File);
   if (HeaderInfo.getHeaderSearchOpts().ImplicitModuleMaps &&
       Known == Headers.end() && File->getDir() == BuiltinIncludeDir &&
-      isBuiltinHeader(llvm::sys::path::filename(File->getName()))) {
+      ModuleMap::isBuiltinHeader(llvm::sys::path::filename(File->getName()))) {
     HeaderInfo.loadTopLevelSystemModules();
     return Headers.find(File);
   }
@@ -1879,7 +1879,7 @@ void ModuleMapParser::parseHeaderDecl(MMToken::TokenKind LeadingToken,
       // supplied by Clang. Find that builtin header.
       if (ActiveModule->IsSystem && LeadingToken != MMToken::UmbrellaKeyword &&
           BuiltinIncludeDir && BuiltinIncludeDir != Directory &&
-          isBuiltinHeader(Header.FileName)) {
+          ModuleMap::isBuiltinHeader(Header.FileName)) {
         SmallString<128> BuiltinPathName(BuiltinIncludeDir->getName());
         llvm::sys::path::append(BuiltinPathName, Header.FileName);
         BuiltinFile = SourceMgr.getFileManager().getFile(BuiltinPathName);
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 9661e7b13f7..322c5809cd2 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1999,6 +1999,7 @@ void Preprocessor::HandleIncludeDirective(SourceLocation HashLoc,
   bool SkipHeader = false;
   if (ShouldEnter &&
       !HeaderInfo.ShouldEnterIncludeFile(*this, File, isImport,
+                                         getLangOpts().Modules,
                                          SuggestedModule.getModule())) {
     ShouldEnter = false;
     SkipHeader = true;