[Clang] Diagnose defaulted assignment operator with incompatible object parameter.

Per https://eel.is/c++draft/dcl.fct.def.default#2.2, the explicit object
parameter of a defaulted special member function must be of the same type
as the one of an equivalent implicitly defaulted function,
ignoring references.

Fixes #69233

Author: cor3ntin

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9483,6 +9483,9 @@ def err_defaulted_special_member_return_type : Error<
 def err_defaulted_special_member_quals : Error<
   "an explicitly-defaulted %select{copy|move}0 assignment operator may not "
   "have 'const'%select{, 'constexpr'|}1 or 'volatile' qualifiers">;
+def err_defaulted_special_member_explicit_object_mismatch : Error<
+  "the type of the explicit object parameter of an explicitly-defaulted "
+  "%select{copy|move}0 assignment operator should match the type of the class %1">;
 def err_defaulted_special_member_volatile_param : Error<
   "the parameter for an explicitly-defaulted %sub{select_special_member_kind}0 "
   "may not be volatile">;

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7748,6 +7748,25 @@ bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
         HadError = true;
       }
     }
+    // [C++23][dcl.fct.def.default]/p2.2
+    // if F2 has an implicit object parameter of type “reference to C”,
+    // F1 may be an explicit object member function whose explicit object
+    // parameter is of (possibly different) type “reference to C”,
+    // in which case the type of F1 would differ from the type of F2
+    // in that the type of F1 has an additional parameter;
+    if (!Context.hasSameType(
+            ThisType.getNonReferenceType().getUnqualifiedType(),
+            Context.getRecordType(RD))) {
+      if (DeleteOnTypeMismatch)
+        ShouldDeleteForTypeMismatch = true;
+      else {
+        Diag(MD->getLocation(),
+             diag::err_defaulted_special_member_explicit_object_mismatch)
+            << (CSM == CXXMoveAssignment) << RD
+            << MD->getSourceRange();
+        HadError = true;
+      }
+    }
   }
 
   // Check for parameter type matching.

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

@@ -585,3 +585,34 @@ class Server : public Thing {
     S name_;
 };
 }
+
+namespace GH69233 {
+struct Base {};
+struct S : Base {
+    int j;
+    S& operator=(this Base& self, const S&) = default;
+    // expected-warning@-1 {{explicitly defaulted copy assignment operator is implicitly deleted}}
+    // expected-note@-2 {{function is implicitly deleted because its declared type does not match the type of an implicit copy assignment operator}}
+    // expected-note@-3 {{explicitly defaulted function was implicitly deleted here}}
+};
+
+struct S2 {
+    S2& operator=(this int&& self, const S2&);
+    S2& operator=(this int&& self, S2&&);
+    operator int();
+};
+
+S2& S2::operator=(this int&& self, const S2&) = default;
+// expected-error@-1 {{the type of the explicit object parameter of an explicitly-defaulted copy assignment operator should match the type of the class 'S2'}}
+
+S2& S2::operator=(this int&& self, S2&&) = default;
+// expected-error@-1 {{the type of the explicit object parameter of an explicitly-defaulted move assignment operator should match the type of the class 'S2'}}
+
+void test() {
+    S s;
+    s = s; // expected-error {{object of type 'S' cannot be assigned because its copy assignment operator is implicitly deleted}}
+    S2 s2;
+    s2 = s2;
+}
+
+}