[c++20] Implement P0846R0: allow (ADL-only) calls to template-ids whose

template name is not visible to unqualified lookup.

In order to support this without a severe degradation in our ability to
diagnose typos in template names, this change significantly restructures
the way we handle template-id-shaped syntax for which lookup of the
template name finds nothing.

Instead of eagerly diagnosing an undeclared template name, we now form a
placeholder template-name representing a name that is known to not find
any templates. When the parser sees such a name, it attempts to
disambiguate whether we have a less-than comparison or a template-id.
Any diagnostics or typo-correction for the name are delayed until its
point of use.

The upshot should be a small improvement of our diagostic quality
overall: we now take more syntactic context into account when trying to
resolve an undeclared identifier on the left hand side of a '<'. In
fact, this works well enough that the backwards-compatible portion (for
an undeclared identifier rather than a lookup that finds functions but
no function templates) is enabled in all language modes.

llvm-svn: 360308

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index f5c404f7e62..9c9a9f46798 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2916,6 +2916,7 @@ Parser::DiagnoseMissingSemiAfterTagDefinition(DeclSpec &DS, AccessSpecifier AS,
       case Sema::NC_Expression:
       case Sema::NC_VarTemplate:
       case Sema::NC_FunctionTemplate:
+      case Sema::NC_UndeclaredTemplate:
         // Might be a redeclaration of a prior entity.
         break;
       }
@@ -3386,7 +3387,8 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // type-name
     case tok::annot_template_id: {
       TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
-      if (TemplateId->Kind != TNK_Type_template) {
+      if (TemplateId->Kind != TNK_Type_template &&
+          TemplateId->Kind != TNK_Undeclared_template) {
         // This template-id does not refer to a type name, so we're
         // done with the type-specifiers.
         goto DoneWithDeclSpec;