[clang] Revert "[Clang] Do not treat Foo -> const Foo conversion sequences as perfect" (PR #149272)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/149272

Reverts llvm/llvm-project#148613

Considering object argument conversion qualifications perfect leads to situations where we prefer a non-template const qualified function over a non-qualified template function, which is very wrong indeed. 

I explored solutions to work around that, but instead, we might want to go the GCC road and prefer the friend overload in the #147374 example, as this seems a lot more consistent and reliable


>From 8a9a1b831888b431771ea65577f588bfdfa56c5a Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Thu, 17 Jul 2025 10:33:44 +0200
Subject: [PATCH] =?UTF-8?q?Revert=20"[Clang]=20Do=20not=20treat=20Foo=20->?=
 =?UTF-8?q?=20const=20Foo=20conversion=20sequences=20as=20perfect=E2=80=A6?=
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This reverts commit acf07dc77cba07d67c85e120c2eb8094438572e9.
---
 clang/include/clang/Sema/Overload.h           | 11 ++------
 clang/lib/Sema/SemaOverload.cpp               | 14 +---------
 ...overload-resolution-deferred-templates.cpp | 28 -------------------
 3 files changed, 4 insertions(+), 49 deletions(-)

diff --git a/clang/include/clang/Sema/Overload.h b/clang/include/clang/Sema/Overload.h
index 9135ff949eeab..a70335bef9dd4 100644
--- a/clang/include/clang/Sema/Overload.h
+++ b/clang/include/clang/Sema/Overload.h
@@ -350,11 +350,6 @@ 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)
@@ -453,11 +448,11 @@ class Sema;
 #endif
         return true;
       }
+      if (!C.hasSameType(getFromType(), getToType(2)))
+        return false;
       if (BindsToRvalue && IsLvalueReference)
         return false;
-      if (IsImplicitObjectArgumentQualificationConversion)
-        return C.hasSameUnqualifiedType(getFromType(), getToType(2));
-      return C.hasSameType(getFromType(), getToType(2));
+      return true;
     }
 
     ImplicitConversionRank getRank() const;
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index f3baf0c3ef3bc..1b54628c5e564 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -245,7 +245,6 @@ void StandardConversionSequence::setAsIdentityConversion() {
   IsLvalueReference = true;
   BindsToFunctionLvalue = false;
   BindsToRvalue = false;
-  IsImplicitObjectArgumentQualificationConversion = false;
   BindsImplicitObjectArgumentWithoutRefQualifier = false;
   ObjCLifetimeConversionBinding = false;
   FromBracedInitList = false;
@@ -5318,7 +5317,6 @@ 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 =
@@ -5498,7 +5496,6 @@ 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()) {
@@ -5521,8 +5518,6 @@ 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;
@@ -5807,7 +5802,6 @@ 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;
@@ -6005,12 +5999,8 @@ static ImplicitConversionSequence TryObjectArgumentInitialization(
   // affects the conversion rank.
   QualType ClassTypeCanon = S.Context.getCanonicalType(ClassType);
   ImplicitConversionKind SecondKind;
-  bool IsQualificationConversion = false;
-  if (ImplicitParamType.getCanonicalType() == FromTypeCanon) {
+  if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
     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()) {
@@ -6051,8 +6041,6 @@ 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;
diff --git a/clang/test/SemaCXX/overload-resolution-deferred-templates.cpp b/clang/test/SemaCXX/overload-resolution-deferred-templates.cpp
index 135865c8450f5..46c3670848529 100644
--- a/clang/test/SemaCXX/overload-resolution-deferred-templates.cpp
+++ b/clang/test/SemaCXX/overload-resolution-deferred-templates.cpp
@@ -283,31 +283,3 @@ 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 at -1 {{use of overloaded operator '+' is ambiguous (with operand types 'Bar' and 'String')}}
-
-    Baz z;
-    z + b;
-    //expected-error at -1 {{use of overloaded operator '+' is ambiguous (with operand types 'Baz' and 'String')}}
-}
-
-}