[Clang][AST] Fix a crash on attaching doc comments

clang/lib/AST/ASTContext.cpp

@@ -498,7 +498,11 @@ void ASTContext::attachCommentsToJustParsedDecls(ArrayRef<Decl *> Decls,
     return;
 
   FileID File;
-  for (Decl *D : Decls) {
+  for (const Decl *D : Decls) {
+    if (D->isInvalidDecl())
+      continue;
+
+    D = &adjustDeclToTemplate(*D);
     SourceLocation Loc = D->getLocation();
     if (Loc.isValid()) {
       // See if there are any new comments that are not attached to a decl.

clang/test/AST/ast-crash-doc-function-template.cpp

@@ -0,0 +1,30 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -x c++ -Wdocumentation -fsyntax-only -ast-dump-all %t/t.cpp
+
+//--- t.h
+/// MyClass in the header file
+class MyClass {
+public:
+  template <typename T>
+  void Foo() const;
+
+  /// Bar
+  void Bar() const;
+};
+
+//--- t.cpp
+#include "t.h"
+
+/// MyClass::Bar: Foo<int>() is implicitly instantiated and called here.
+void MyClass::Bar() const {
+  Foo<int>();
+}
+
+/// MyClass::Foo
+template <typename T>
+void MyClass::Foo() const {
+}
+
+// CHECK: TranslationUnitDecl