[clang] [clang] Improve bit-field in ref NTTP diagnostic (PR #71077)

[Andrey Ali Khan Bolshakov via cfe-commits]

https://github.com/bolshakov-a updated https://github.com/llvm/llvm-project/pull/71077

>From 02154cf040c55b0d7bd28a3ffbb76c86ca9b878d Mon Sep 17 00:00:00 2001
From: Bolshakov <bolsh.andrey at yandex.ru>
Date: Thu, 2 Nov 2023 19:20:27 +0300
Subject: [PATCH] [clang] Improve bit-field in ref NTTP diagnostic

Prior to this, attempts to bind a bit-field to an NTTP of reference type
produced an error because references to subobjects in NTTPs are
disallowed. But C++20 allows references to subobjects in NTTPs generally
(see P1907R1). Without this change, implementing P1907R1 would cause
a bug allowing bit-fields to be bound to reference template arguments.

Extracted from https://reviews.llvm.org/D140996
---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 2 ++
 clang/lib/Sema/SemaOverload.cpp                  | 9 +++++++++
 clang/test/CXX/drs/dr12xx.cpp                    | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 474afc2fb99c1f0..94b836252eda571 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2251,6 +2251,8 @@ def warn_cxx17_compat_aggregate_init_paren_list : Warning<
 def err_reference_bind_to_bitfield : Error<
   "%select{non-const|volatile}0 reference cannot bind to "
   "bit-field%select{| %1}2">;
+def err_reference_bind_to_bitfield_in_cce : Error<
+  "reference cannot bind to bit-field in converted constant expression">;
 def err_reference_bind_to_vector_element : Error<
   "%select{non-const|volatile}0 reference cannot bind to vector element">;
 def err_reference_bind_to_matrix_element : Error<
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index d3d2dfed2ce0cc2..36390481e1ce6d4 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -5992,6 +5992,15 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
                                   /*InOverloadResolution=*/false,
                                   /*AllowObjCWritebackConversion=*/false,
                                   /*AllowExplicit=*/false);
+
+  // TryCopyInitialization returns incorrect info for attempts to bind reference
+  // to bit-field due to C++ [over.ics.ref]p4, so check it here.
+  if (From->refersToBitField() && T.getTypePtr()->isReferenceType()) {
+    return S.Diag(From->getBeginLoc(),
+                  diag::err_reference_bind_to_bitfield_in_cce)
+           << From->getSourceRange();
+  }
+
   StandardConversionSequence *SCS = nullptr;
   switch (ICS.getKind()) {
   case ImplicitConversionSequence::StandardConversion:
diff --git a/clang/test/CXX/drs/dr12xx.cpp b/clang/test/CXX/drs/dr12xx.cpp
index c23a515ba56cb99..81c113d4c1c5ac7 100644
--- a/clang/test/CXX/drs/dr12xx.cpp
+++ b/clang/test/CXX/drs/dr12xx.cpp
@@ -138,7 +138,7 @@ namespace dr1295 { // dr1295: 4
 #if __cplusplus <= 201402L
   // expected-error at -2 {{does not refer to any declaration}} expected-note at -3 {{here}}
 #else
-  // expected-error at -4 {{refers to subobject}}
+  // expected-error at -4 {{reference cannot bind to bit-field in converted constant expression}}
 #endif
 
 #if __cplusplus >= 201103L