Raise an error on namespace aliases with qualified names.

Current behavior is to ignore the trailing qualifiers, but the grammar
for `namespace-alias-definition` requires an `identifier` without
qualification.

https://godbolt.org/z/1zvW5q4f8

https://eel.is/c++draft/namespace.alias#nt:namespace-alias-definition

Author: katzdm

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -268,6 +268,8 @@ def err_expected_semi_after_namespace_name : Error<
   "expected ';' after namespace name">;
 def err_unexpected_namespace_attributes_alias : Error<
   "attributes cannot be specified on namespace alias">;
+def err_unexpected_qualified_namespace_alias : Error<
+  "unexpected nested name specifier in namespace alias definition">;
 def err_unexpected_nested_namespace_attribute : Error<
   "attributes cannot be specified on a nested namespace definition">;
 def err_inline_namespace_alias : Error<"namespace alias cannot be inline">;

clang/lib/Parse/ParseDeclCXX.cpp

@@ -140,6 +140,11 @@ Parser::DeclGroupPtrTy Parser::ParseNamespace(DeclaratorContext Context,
       SkipUntil(tok::semi);
       return nullptr;
     }
+    if (!ExtraNSs.empty()) {
+      Diag(IdentLoc, diag::err_unexpected_qualified_namespace_alias);
+      SkipUntil(tok::semi);
+      return nullptr;
+    }
     if (attrLoc.isValid())
       Diag(attrLoc, diag::err_unexpected_namespace_attributes_alias);
     if (InlineLoc.isValid())

clang/test/SemaCXX/namespace-alias.cpp

@@ -47,6 +47,8 @@ namespace I {
   namespace A1 { int i; }
 
   namespace A2 = A1;
+
+  namespace A3::extra::specifiers = A2;  // expected-error {{unexpected nested name specifier}}
 }
 
 int f() {