[Clang] Fix deduction of explicit object member functions (#140030)

When taking the address of an overload set containing an explicit object
member, we should not take the
explicit object parameter into account.

clang/docs/ReleaseNotes.rst

@@ -710,6 +710,7 @@ Bug Fixes to C++ Support
 - Clang now correctly parses arbitrary order of ``[[]]``, ``__attribute__`` and ``alignas`` attributes for declarations (#GH133107)
 - Fixed a crash when forming an invalid function type in a dependent context. (#GH138657) (#GH115725) (#GH68852)
 - Clang no longer segfaults when there is a configuration mismatch between modules and their users (http://crbug.com/400353616).
+- Fix an incorrect deduction when calling an explicit object member function template through an overload set address.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -12576,6 +12576,7 @@ public:
       bool PartialOverloading, bool AggregateDeductionCandidate,
       bool PartialOrdering, QualType ObjectType,
       Expr::Classification ObjectClassification,
+      bool ForOverloadSetAddressResolution,
       llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent);
 
   /// Deduce template arguments when taking the address of a function

clang/lib/Sema/SemaOverload.cpp

@@ -7846,6 +7846,8 @@ static void AddMethodTemplateCandidateImmediately(
           MethodTmpl, ExplicitTemplateArgs, Args, Specialization, Info,
           PartialOverloading, /*AggregateDeductionCandidate=*/false,
           /*PartialOrdering=*/false, ObjectType, ObjectClassification,
+          CandidateSet.getKind() ==
+              clang::OverloadCandidateSet::CSK_AddressOfOverloadSet,
           [&](ArrayRef<QualType> ParamTypes) {
             return S.CheckNonDependentConversions(
                 MethodTmpl, ParamTypes, Args, CandidateSet, Conversions,
@@ -7960,6 +7962,8 @@ static void AddTemplateOverloadCandidateImmediately(
           /*PartialOrdering=*/false,
           /*ObjectType=*/QualType(),
           /*ObjectClassification=*/Expr::Classification(),
+          CandidateSet.getKind() ==
+              OverloadCandidateSet::CSK_AddressOfOverloadSet,
           [&](ArrayRef<QualType> ParamTypes) {
             return S.CheckNonDependentConversions(
                 FunctionTemplate, ParamTypes, Args, CandidateSet, Conversions,

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -4432,6 +4432,7 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
     bool PartialOverloading, bool AggregateDeductionCandidate,
     bool PartialOrdering, QualType ObjectType,
     Expr::Classification ObjectClassification,
+    bool ForOverloadSetAddressResolution,
     llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent) {
   if (FunctionTemplate->isInvalidDecl())
     return TemplateDeductionResult::Invalid;
@@ -4440,7 +4441,15 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
   unsigned NumParams = Function->getNumParams();
   bool HasExplicitObject = false;
   int ExplicitObjectOffset = 0;
-  if (Function->hasCXXExplicitFunctionObjectParameter()) {
+
+  // [C++26] [over.call.func]p3
+  // If the primary-expression is the address of an overload set,
+  // the argument list is the same as the expression-list in the call.
+  // Otherwise, the argument list is the expression-list in the call augmented
+  // by the addition of an implied object argument as in a qualified function
+  // call.
+  if (!ForOverloadSetAddressResolution &&
+      Function->hasCXXExplicitFunctionObjectParameter()) {
     HasExplicitObject = true;
     ExplicitObjectOffset = 1;
   }

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -926,6 +926,33 @@ struct C {
     (&fref)();
   }
 };
+
+struct CTpl {
+  template <typename T>
+  constexpr int c(this const CTpl&, T) {  // #P2797-ctpl-1
+      return 42;
+  }
+
+  template <typename T>
+  void c(T)&; // #P2797-ctpl-2
+
+  template <typename T>
+  static void c(T = 0, T = 0);  // #P2797-ctpl-3
+
+  void d() {
+    c(0);               // expected-error {{call to member function 'c' is ambiguous}}
+                        // expected-note@#P2797-ctpl-1{{candidate}}
+                        // expected-note@#P2797-ctpl-2{{candidate}}
+                        // expected-note@#P2797-ctpl-3{{candidate}}
+    (CTpl::c)(0);       // expected-error {{call to member function 'c' is ambiguous}}
+                        // expected-note@#P2797-ctpl-1{{candidate}}
+                        // expected-note@#P2797-ctpl-2{{candidate}}
+                        // expected-note@#P2797-ctpl-3{{candidate}}
+
+    static_assert((&CTpl::c)(CTpl{}, 0) == 42); // selects #1
+  }
+};
+
 }
 
 namespace GH85992 {