[Sema]Use tag name lookup for class names

clang/docs/ReleaseNotes.rst

@@ -516,6 +516,7 @@ Bug Fixes to C++ Support
 - Fixed a bug in lambda captures where ``constexpr`` class-type objects were not properly considered ODR-used in
   certain situations. (#GH47400), (#GH90896)
 - Fix erroneous templated array size calculation leading to crashes in generated code. (#GH41441)
+- During the lookup for a base class name, non-type names are ignored. (#GH16855)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -357,10 +357,13 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
       return nullptr;
   }
 
-  // FIXME: LookupNestedNameSpecifierName isn't the right kind of
-  // lookup for class-names.
-  LookupNameKind Kind = isClassName ? LookupNestedNameSpecifierName :
-                                      LookupOrdinaryName;
+  // In the case where we know that the identifier is a class name, we know that
+  // it is a type declaration (struct, class, union or enum) so we can use tag
+  // name lookup.
+  //
+  // C++ [class.derived]p2 (wrt lookup in a base-specifier): The lookup for
+  // the component name of the type-name or simple-template-id is type-only.
+  LookupNameKind Kind = isClassName ? LookupTagName : LookupOrdinaryName;
   LookupResult Result(*this, &II, NameLoc, Kind);
   if (LookupCtx) {
     // Perform "qualified" name lookup into the declaration context we

clang/test/CXX/class.derived/p2.cpp

@@ -6,4 +6,16 @@ namespace PR5840 {
   struct Base {};
   int Base = 10;
   struct Derived : Base {};
-}
+} // namespace PR5840
+
+namespace issue_16855 {
+  struct x {};
+  namespace
+  {
+      namespace x
+      {
+          struct y : x
+          {};
+      } // namespace x
+  }
+} // namespace issue_16855