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

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Andrey Ali Khan Bolshakov (bolshakov-a)

<details>
<summary>Changes</summary>

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](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1907r1.html)). 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

---
Full diff: https://github.com/llvm/llvm-project/pull/71077.diff


3 Files Affected:

- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2) 
- (modified) clang/lib/Sema/SemaOverload.cpp (+9) 
- (modified) clang/test/CXX/drs/dr12xx.cpp (+1-1) 


``````````diff
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..ac28d99784d9c5a 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 {{bind to bit-field in converted constant expression}}
 #endif
 
 #if __cplusplus >= 201103L

``````````

</details>


https://github.com/llvm/llvm-project/pull/71077