[clang] Reject auto combined with type specifiers in C++ (PR #208552)

[Tony Guillot via cfe-commits]

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@@ -3860,18 +3862,44 @@ void Parser::ParseDeclarationSpecifiers(
                                   DS.isFriendSpecified()))
         goto DoneWithDeclSpec;
 
+      // If 'auto' is set and we're in a template parameter context, the
+      // identifier is always the parameter name, not a type specifier, so skip
+      // type name lookup to avoid false ambiguity errors.
+      if (DS.getTypeSpecType() == DeclSpec::TST_auto &&
+          DSContext == DeclSpecContext::DSC_template_param) {
+        goto DoneWithDeclSpec;
+      }
+
+      // If 'auto' is set and the next token indicates this identifier is the
+      // declarator-id, stop parsing declaration specifiers before doing type
+      // lookup. Looking up the declarator-id can produce bogus ambiguity errors
+      // when a variable name matches a type brought in by a using-directive.
+      if (DS.getTypeSpecType() == DeclSpec::TST_auto) {
+        Token Next = NextToken();
+        if (Next.isOneOf(tok::equal, tok::l_paren, tok::l_square, tok::amp,
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to268 wrote:

It looks like there is an issue with this patch. I will send a patch to fix the issue.

https://github.com/llvm/llvm-project/pull/208552