Emit a diagnostic warning about parsing ambiguity in a conjunction

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1713,6 +1713,10 @@ let CategoryName = "Reflection Issue" in {
 def err_cannot_reflect_entity : Error<"cannot reflect entity">;
 def err_expansion_statements_disabled : Error<
   "'template for' statements are not enabled; use '-fexpansion-statements'">;
+
+def warn_parsing_ambiguity_in_refl_expression_with_ampamp_token : Warning <
+    "the compound condition may be misinterpreted due to '%0 &&' type parsing logic. did you mean '... (^%0) && ...'?"
+    >;
 }
 
 let CategoryName = "Generics Issue" in {

clang/lib/Parse/ParseDecl.cpp

@@ -6750,12 +6750,24 @@ void Parser::ParseDeclaratorInternal(Declarator &D,
     // Is a reference
     DeclSpec DS(AttrFactory);
 
-    // Complain about rvalue references in C++03, but then go on and build
-    // the declarator.
-    if (Kind == tok::ampamp)
-      Diag(Loc, getLangOpts().CPlusPlus11 ?
-           diag::warn_cxx98_compat_rvalue_reference :
-           diag::ext_rvalue_reference);
+    // Complain about:
+    // - rvalue references in C++03
+    // - `^T &&` parsing ambiguity of compound expression with reflection in C++2x
+    // but then go on and build the declarator.
+    if (Kind == tok::ampamp) {
+      if (D.getContext() == DeclaratorContext::ReflectOperator) {
+        // parser already consumed '^' token before setting this context
+        Diag(Loc,
+             diag::warn_parsing_ambiguity_in_refl_expression_with_ampamp_token)
+            << (D.hasName()
+                    ? Actions.GetNameForDeclarator(D).getName().getAsString()
+                    : "T");
+      }
+
+      Diag(Loc, getLangOpts().CPlusPlus11
+                    ? diag::warn_cxx98_compat_rvalue_reference
+                    : diag::ext_rvalue_reference);
+    }
 
     // GNU-style and C++11 attributes are allowed here, as is restrict.
     ParseTypeQualifierListOpt(DS);

clang/test/Parser/cxx2x-ambig-reflect-expr.cpp

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// Copyright 2024 Bloomberg Finance L.P.
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// RUN: %clang_cc1 -std=c++26 -freflection -verify %s
+
+
+enum MyEnum { x, y, e = -1, f, z = 99 };
+
+void func(MyEnum && x) { // ok
+  MyEnum value{};
+  MyEnum& ref = value;
+
+  constexpr auto reflValue = ^value;
+
+  constexpr bool test_comparison_0 = reflValue != (^MyEnum) && true; // ok
+  constexpr bool test_comparison_1 = (reflValue != (^MyEnum) && true); // ok
+
+  constexpr bool test_comparison_2 = ^MyEnum != (^MyEnum) && true; // ok
+  constexpr bool test_comparison_3 = (^MyEnum != (^MyEnum) && true); // ok
+
+  constexpr bool test_comparison_4 = reflValue != ^MyEnum && true;
+  // expected-warning@28 {{the compound condition may be misinterpreted due to 'T &&' type parsing logic. did you mean '... (^T) && ...'?}}
+  // expected-error@28 {{expected ';' at end of declaration}}
+}
+