[Clang] Fix confusing diagnositcs related to explicit this parameters

clang/docs/ReleaseNotes.rst

@@ -124,6 +124,8 @@ Improvements to Clang's diagnostics
       template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
      };
 
+- Clang now has improved diagnostics for functions with explicit 'this' parameters. Fixes #GH97878
+
 - Clang now diagnoses dangling references to fields of temporary objects. Fixes #GH81589.
 
 - Clang now diagnoses undefined behavior in constant expressions more consistently. This includes invalid shifts, and signed overflow in arithmetic.

clang/lib/Sema/SemaOverload.cpp

@@ -11133,7 +11133,7 @@ static void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand,
   if (isa<CXXMethodDecl>(Fn) && !isa<CXXConstructorDecl>(Fn)) {
     if (I == 0)
       isObjectArgument = true;
-    else
+    else if (!Fn->hasCXXExplicitFunctionObjectParameter())
       I--;
   }
 

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

@@ -965,3 +965,10 @@ void f();
 };
 void a::f(this auto) {} // expected-error {{an explicit object parameter cannot appear in a non-member function}}
 }
+
+struct R {
+  void f(this auto &&self, int &&r_value_ref) {} // expected-note {{candidate function template not viable: expects an rvalue for 2nd argument}}
+  void g(int &&r_value_ref) {
+	f(r_value_ref); // expected-error {{no matching member function for call to 'f'}}
+  }
+};