[Sema] Fix a crash when attaching comments to an implicit decl

When an implicitly generated decl was the first entry in the group, we
attempted to lookup comments with an empty FileID, leading to crashes. Avoid
this by trying to use the other declarations in the group, and then bailing out
if none are valid.

rdar://59919733
Differential revision: https://reviews.llvm.org/D75483

clang/lib/AST/ASTContext.cpp

@@ -474,10 +474,20 @@ void ASTContext::attachCommentsToJustParsedDecls(ArrayRef<Decl *> Decls,
   if (Comments.empty() || Decls.empty())
     return;
 
-  // See if there are any new comments that are not attached to a decl.
-  // The location doesn't have to be precise - we care only about the file.
-  const FileID File =
-      SourceMgr.getDecomposedLoc((*Decls.begin())->getLocation()).first;
+  FileID File;
+  for (Decl *D : Decls) {
+    SourceLocation Loc = D->getLocation();
+    if (Loc.isValid()) {
+      // See if there are any new comments that are not attached to a decl.
+      // The location doesn't have to be precise - we care only about the file.
+      File = SourceMgr.getDecomposedLoc(Loc).first;
+      break;
+    }
+  }
+
+  if (File.isInvalid())
+    return;
+
   auto CommentsInThisFile = Comments.getCommentsInFile(File);
   if (!CommentsInThisFile || CommentsInThisFile->empty() ||
       CommentsInThisFile->rbegin()->second->isAttached())

clang/test/Sema/warn-documentation.m

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -fblocks -Wno-objc-root-class -Wdocumentation -Wdocumentation-pedantic -verify %s
+// RUN: %clang_cc1 -xobjective-c++ -fsyntax-only -fblocks -Wno-objc-root-class -Wdocumentation -Wdocumentation-pedantic -verify %s
 
 @class NSString;
 
@@ -318,3 +319,10 @@ @interface CheckFunctionBlockPointerVars {
 // expected-warning@-1 {{'\return' command used in a comment that is not attached to a function or method declaration}}
 VoidBlockTypeCall ^e; ///< \return none
 // expected-warning@-1 {{'\return' command used in a comment that is not attached to a function or method declaration}}
+
+#ifdef __cplusplus
+@interface HasAnonNamespace @end
+@implementation HasAnonNamespace
+namespace {}
+@end
+#endif