[Clang] Fix a crash when incorrectly calling an explicit object member function template (#75913)

Fixes #75732

Author: cor3ntin

clang/lib/Sema/SemaExprMember.cpp

@@ -253,7 +253,9 @@ static void diagnoseInstanceReference(Sema &SemaRef,
     SemaRef.Diag(Loc, diag::err_member_call_without_object)
         << Range << /*static*/ 0;
   else {
-    const auto *Callee = dyn_cast<CXXMethodDecl>(Rep);
+    if (const auto *Tpl = dyn_cast<FunctionTemplateDecl>(Rep))
+      Rep = Tpl->getTemplatedDecl();
+    const auto *Callee = cast<CXXMethodDecl>(Rep);
     auto Diag = SemaRef.Diag(Loc, diag::err_member_call_without_object)
                 << Range << Callee->isExplicitObjectMemberFunction();
     if (!Replacement.empty())

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

@@ -626,3 +626,13 @@ void test() {
 }
 
 }
+
+
+namespace GH75732 {
+auto serialize(auto&& archive, auto&& c){ }
+struct D {
+    auto serialize(this auto&& self, auto&& archive) {
+        serialize(archive, self); // expected-error {{call to explicit member function without an object argument}}
+    }
+};
+}