[clangd] Fix an assertion failure in Selection.

Summary:
The assertion is triggered when the Decl is null.

Details for the assertion:

F0207 09:55:09.069385   47308 logging.cc:84] assert.h assertion failed at llvm/include/llvm/Support/Casting.h:105 in static bool llvm::isa_impl_cl<clang::TranslationUnitDecl, const clang::      Decl *>::doit(const From *) [To = clang::TranslationUnitDecl, From = const clang::Decl *]: Val && "isa<> used on a null pointer"
 15 *** Check failure stack trace: ***
 19     @     0x55615c1f7e06  __assert_fail
 20     @     0x55615a6297d8  clang::clangd::(anonymous namespace)::SelectionVisitor::TraverseDecl()
 21     @     0x55615a62f48d  clang::RecursiveASTVisitor<>::TraverseTemplateTemplateParmDecl()
 22     @     0x55615a62b264  clang::RecursiveASTVisitor<>::TraverseDecl()
 23     @     0x55615a62979c  clang::clangd::(anonymous namespace)::SelectionVisitor::TraverseDecl()
 24     @     0x55615a63060c  clang::RecursiveASTVisitor<>::TraverseClassTemplatePartialSpecializationDecl()
 25     @     0x55615a62ae45  clang::RecursiveASTVisitor<>::TraverseDecl()

Reviewers: sammccall

Subscribers: javed.absar, kristof.beyls, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57879

llvm-svn: 353421

2 files changed, 11 insertions, 1 deletions

diff --git a/clang-tools-extra/clangd/Selection.cpp b/clang-tools-extra/clangd/Selection.cpp
index 2c2fd476072..d1ea30e7ede 100644
--- a/clang-tools-extra/clangd/Selection.cpp
+++ b/clang-tools-extra/clangd/Selection.cpp
@@ -51,7 +51,7 @@ public:
   //  - those that can't be stored in DynTypedNode.
   // We're missing some interesting things like Attr due to the latter.
   bool TraverseDecl(Decl *X) {
-    if (isa<TranslationUnitDecl>(X))
+    if (X && isa<TranslationUnitDecl>(X))
       return Base::TraverseDecl(X); // Already pushed by constructor.
     return traverseNode(X, [&] { return Base::TraverseDecl(X); });
   }
diff --git a/clang-tools-extra/unittests/clangd/SelectionTests.cpp b/clang-tools-extra/unittests/clangd/SelectionTests.cpp
index 88406b6331e..2322a0e6a54 100644
--- a/clang-tools-extra/unittests/clangd/SelectionTests.cpp
+++ b/clang-tools-extra/unittests/clangd/SelectionTests.cpp
@@ -176,6 +176,16 @@ TEST(SelectionTest, CommonAncestor) {
 
       // Node types that have caused problems in the past.
       {"template <typename T> void foo() { [[^T]] t; }", "TypeLoc"},
+
+      // No crash
+      {
+          R"cpp(
+            template <class T> struct Foo {};
+            template <[[template<class> class /*cursor here*/^U]]>
+             struct Foo<U<int>*> {};
+          )cpp",
+          "TemplateTemplateParmDecl"
+      },
   };
   for (const Case &C : Cases) {
     Annotations Test(C.Code);