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

[erik.pilkington]

Summary

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 a group, then bailing out if none are valid.

rdar://59919733

[jkorous]

Thanks a bunch for fixing this!

That also means we can skip all the implicit declarations from the for (const Decl *D : Decls) { just a couple lines below your fix since implicit declaration can hardly have a doc comment attached... I'll send a patch.

clang/lib/AST/ASTContext.cpp

static void addRedeclaredMethods(const ObjCMethodDecl *ObjCMethod,
   }
 }
 
 void ASTContext::attachCommentsToJustParsedDecls(ArrayRef<Decl *> Decls,
                                                  const Preprocessor *PP) {
   if (Comments.empty() || Decls.empty())
     return;
 
+  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.
-  const FileID File =
-      SourceMgr.getDecomposedLoc((*Decls.begin())->getLocation()).first;
+      File = SourceMgr.getDecomposedLoc(Loc).first;
+      break;
+    }
+  }
+
+  if (File.isInvalid())
+    return;
+
   auto CommentsInThisFile = Comments.getCommentsInFile(File);
   if (!CommentsInThisFile || CommentsInThisFile->empty() ||
       CommentsInThisFile->rbegin()->second->isAttached())
     return;
 
   // There is at least one comment not attached to a decl.
   // Maybe it should be attached to one of Decls?
   //

clang/test/Sema/warn-documentation.m

 // 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;
 
 // expected-warning@+1 {{empty paragraph passed to '\brief' command}}
 /// \brief\author Aaa
 @interface Test1
 // expected-warning@+2 {{empty paragraph passed to '\brief' command}}
 /**

 
 // PR42844 - Assertion failures when using typedefed block pointers
 typedef void(^VoidBlockType)();
 typedef VoidBlockType VoidBlockTypeCall();
 VoidBlockTypeCall *d; ///< \return none
 // 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