[C++] Implement "Deducing this" (P0847R7)

This patch implements P0847R7 (partially),
CWG2561 and CWG2653.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D140828

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 03c19bf..8b653b1 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1571,6 +1571,17 @@ Parser::isCXXDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
   case tok::kw___vector:
     return TPResult::True;
 
+  case tok::kw_this: {
+    // Try to parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    if (getLangOpts().CPlusPlus) {
+      RevertingTentativeParsingAction PA(*this);
+      ConsumeToken();
+      return isCXXDeclarationSpecifier(AllowImplicitTypename, BracedCastResult,
+                                       InvalidAsDeclSpec);
+    }
+    return TPResult::False;
+  }
   case tok::annot_template_id: {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     // If lookup for the template-name found nothing, don't assume we have a