[Clang] Do not treat Foo -> const Foo conversion sequences as perfect (#148613)

For implicit object arguments. This fixes a regression introduced by the "perfect match" overload resolution mechanism introduced in 8c5a307. Note that GCC allows the ambiguity between a const and non-const candidate to be resolved. But this patch focuses on restoring the Clang 20 behavior, and to fix the cases which we did resolve incorrectly. Fixes #147374

Author: cor3ntin

clang/include/clang/Sema/Overload.h

@@ -350,6 +350,11 @@ class Sema;
  LLVM_PREFERRED_TYPE(bool)
  unsigned BindsToRvalue : 1;
 
+ /// Whether this was an identity conversion with qualification
+ /// conversion for the implicit object argument.
+ LLVM_PREFERRED_TYPE(bool)
+ unsigned IsImplicitObjectArgumentQualificationConversion : 1;
+
  /// Whether this binds an implicit object argument to a
  /// non-static member function without a ref-qualifier.
  LLVM_PREFERRED_TYPE(bool)
@@ -448,11 +453,11 @@ class Sema;
 #endif
  return true;
  }
- if (!C.hasSameType(getFromType(), getToType(2)))
- return false;
  if (BindsToRvalue && IsLvalueReference)
  return false;
- return true;
+ if (IsImplicitObjectArgumentQualificationConversion)
+ return C.hasSameUnqualifiedType(getFromType(), getToType(2));
+ return C.hasSameType(getFromType(), getToType(2));
  }
 
  ImplicitConversionRank getRank() const;

clang/lib/Sema/SemaOverload.cpp

@@ -245,6 +245,7 @@ void StandardConversionSequence::setAsIdentityConversion() {
  IsLvalueReference = true;
  BindsToFunctionLvalue = false;
  BindsToRvalue = false;
+ IsImplicitObjectArgumentQualificationConversion = false;
  BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ObjCLifetimeConversionBinding = false;
  FromBracedInitList = false;
@@ -5317,6 +5318,7 @@ TryReferenceInit(Sema &S, Expr *Init, QualType DeclType,
  ICS.Standard.DirectBinding = BindsDirectly;
  ICS.Standard.IsLvalueReference = !isRValRef;
  ICS.Standard.BindsToFunctionLvalue = T2->isFunctionType();
+ ICS.Standard.IsImplicitObjectArgumentQualificationConversion = false;
  ICS.Standard.BindsToRvalue = InitCategory.isRValue();
  ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ICS.Standard.ObjCLifetimeConversionBinding =
@@ -5496,6 +5498,7 @@ TryReferenceInit(Sema &S, Expr *Init, QualType DeclType,
  ICS.Standard.IsLvalueReference = !isRValRef;
  ICS.Standard.BindsToFunctionLvalue = false;
  ICS.Standard.BindsToRvalue = true;
+ ICS.Standard.IsImplicitObjectArgumentQualificationConversion = false;
  ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ICS.Standard.ObjCLifetimeConversionBinding = false;
  } else if (ICS.isUserDefined()) {
@@ -5518,6 +5521,8 @@ TryReferenceInit(Sema &S, Expr *Init, QualType DeclType,
  ICS.UserDefined.After.IsLvalueReference = !isRValRef;
  ICS.UserDefined.After.BindsToFunctionLvalue = false;
  ICS.UserDefined.After.BindsToRvalue = !LValRefType;
+ ICS.UserDefined.After.IsImplicitObjectArgumentQualificationConversion =
+ false;
  ICS.UserDefined.After.BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ICS.UserDefined.After.ObjCLifetimeConversionBinding = false;
  ICS.UserDefined.After.FromBracedInitList = false;
@@ -5802,6 +5807,7 @@ TryListConversion(Sema &S, InitListExpr *From, QualType ToType,
  StandardConversionSequence &SCS = Result.isStandard() ? Result.Standard :
  Result.UserDefined.After;
  SCS.ReferenceBinding = true;
+ SCS.IsImplicitObjectArgumentQualificationConversion = false;
  SCS.IsLvalueReference = ToType->isLValueReferenceType();
  SCS.BindsToRvalue = true;
  SCS.BindsToFunctionLvalue = false;
@@ -5999,8 +6005,12 @@ static ImplicitConversionSequence TryObjectArgumentInitialization(
  // affects the conversion rank.
  QualType ClassTypeCanon = S.Context.getCanonicalType(ClassType);
  ImplicitConversionKind SecondKind;
- if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
+ bool IsQualificationConversion = false;
+ if (ImplicitParamType.getCanonicalType() == FromTypeCanon) {
  SecondKind = ICK_Identity;
+ } else if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
+ SecondKind = ICK_Identity;
+ IsQualificationConversion = true;
  } else if (S.IsDerivedFrom(Loc, FromType, ClassType)) {
  SecondKind = ICK_Derived_To_Base;
  } else if (!Method->isExplicitObjectMemberFunction()) {
@@ -6041,6 +6051,8 @@ static ImplicitConversionSequence TryObjectArgumentInitialization(
  ICS.Standard.setFromType(FromType);
  ICS.Standard.setAllToTypes(ImplicitParamType);
  ICS.Standard.ReferenceBinding = true;
+ ICS.Standard.IsImplicitObjectArgumentQualificationConversion =
+ IsQualificationConversion;
  ICS.Standard.DirectBinding = true;
  ICS.Standard.IsLvalueReference = Method->getRefQualifier() != RQ_RValue;
  ICS.Standard.BindsToFunctionLvalue = false;

clang/test/SemaCXX/overload-resolution-deferred-templates.cpp

@@ -283,3 +283,31 @@ void f() {
 }
 
 #endif
+
+namespace GH147374 {
+
+struct String {};
+template <typename T> void operator+(T, String &&) = delete;
+
+struct Bar {
+ void operator+(String) const; // expected-note {{candidate function}}
+ friend void operator+(Bar, String) {}; // expected-note {{candidate function}}
+};
+
+struct Baz {
+ void operator+(String); // expected-note {{candidate function}}
+ friend void operator+(Baz, String) {}; // expected-note {{candidate function}}
+};
+
+void test() {
+ Bar a;
+ String b;
+ a + b;
+ //expected-error@-1 {{use of overloaded operator '+' is ambiguous (with operand types 'Bar' and 'String')}}
+
+ Baz z;
+ z + b;
+ //expected-error@-1 {{use of overloaded operator '+' is ambiguous (with operand types 'Baz' and 'String')}}
+}
+
+}