[Concepts] Improve diagnostics on a missing 'auto' keyword.

As reported in https://github.com/llvm/llvm-project/issues/49192,
we did a pretty poor job diagnosing cases where someone forgot 'auto', a
nd is probably in the middle of a variable declaration.  This patch
makes us properly diagnose in cases where the next token is a reference,
or CVR qualifier.

1 files changed, 9 insertions, 2 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index fd35898..d7942fa 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3706,11 +3706,18 @@ void Parser::ParseDeclarationSpecifiers(
 
       if (TemplateId->Kind == TNK_Concept_template) {
         // If we've already diagnosed that this type-constraint has invalid
-        // arguemnts, drop it and just form 'auto' or 'decltype(auto)'.
+        // arguments, drop it and just form 'auto' or 'decltype(auto)'.
         if (TemplateId->hasInvalidArgs())
           TemplateId = nullptr;
 
-        if (NextToken().is(tok::identifier)) {
+        // Any of the following tokens are likely the start of the user
+        // forgetting 'auto' or 'decltype(auto)', so diagnose.
+        // Note: if updating this list, please make sure we update
+        // isCXXDeclarationSpecifier's check for IsPlaceholderSpecifier to have
+        // a matching list.
+        if (NextToken().isOneOf(tok::identifier, tok::kw_const,
+                                tok::kw_volatile, tok::kw_restrict, tok::amp,
+                                tok::ampamp)) {
           Diag(Loc, diag::err_placeholder_expected_auto_or_decltype_auto)
               << FixItHint::CreateInsertion(NextToken().getLocation(), "auto");
           // Attempt to continue as if 'auto' was placed here.