Use tag name lookup for class names

This PR would fix #16855 .

I think the correct lookup to use for class names is Tag name lookup,
because it does not take namespaces into account. The current lookup
does and because of this some valid programs are not accepted.

If you think that Tag name lookup is not correct for all cases when
we are looking up types based on class names then we can only
do tag name lookup when looking up class names for inheritance.
In case of inheritance:
```
[class.derived]p2 says:

"During the lookup for a base class name, non-type names are ignored."
```

Author: spaits

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