[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 e5ad8f5b00a297ef745e1b9341df32b80a7c6b77 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/docs/ReleaseNotes.rst                        |  2 ++
 clang/include/clang/Basic/DiagnosticSemaKinds.td   |  2 ++
 clang/lib/Sema/SemaOverload.cpp                    | 10 ++++++++++
 clang/test/CXX/drs/dr12xx.cpp                      |  2 +-
 clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp |  6 ++++++
 5 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c7a948fd3fae5..01a927b2bc17e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -503,6 +503,8 @@ Improvements to Clang's diagnostics
        48 | static_assert(1 << 4 == 15);
           |               ~~~~~~~^~~~~
 
+- Clang now diagnoses attempts to bind a bitfield to an NTTP of a reference type as erroneous
+  converted constant expression and not as a reference to subobject.
 
 Improvements to Clang's time-trace
 ----------------------------------
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 6dfb2d7195203..b95d2bea00a5e 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 5026e1d603e5e..ebbd4f48b57f8 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -6056,6 +6056,16 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
                   diag::err_typecheck_converted_constant_expression_indirect)
            << From->getType() << From->getSourceRange() << T;
   }
+  // 'TryCopyInitialization' returns incorrect info for attempts to bind
+  // a reference to a bit-field due to C++ [over.ics.ref]p4. Namely,
+  // 'SCS->DirectBinding' occurs to be set to 'true' despite it is not
+  // the direct binding according to C++ [dcl.init.ref]p5. Hence, check this
+  // case explicitly.
+  if (From->refersToBitField() && T.getTypePtr()->isReferenceType()) {
+    return S.Diag(From->getBeginLoc(),
+                  diag::err_reference_bind_to_bitfield_in_cce)
+           << From->getSourceRange();
+  }
 
   // Usually we can simply apply the ImplicitConversionSequence we formed
   // earlier, but that's not guaranteed to work when initializing an object of
diff --git a/clang/test/CXX/drs/dr12xx.cpp b/clang/test/CXX/drs/dr12xx.cpp
index c23a515ba56cb..81c113d4c1c5a 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
diff --git a/clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp b/clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
index 792dc78464b2a..982f6ec221570 100644
--- a/clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
+++ b/clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
@@ -93,6 +93,12 @@ namespace ConvertedConstant {
   template <A> struct X {};
   void f(X<1.0f>) {}
   void g(X<2>) {}
+
+  struct {
+    int i : 2;
+  } b;
+  template <const int&> struct Y {};
+  void f(Y<b.i>) {} // expected-error {{reference cannot bind to bit-field in converted constant expression}}
 }
 
 namespace CopyCounting {