Rebase, extend the test a bit

[Fznamznon]

Miss click!

clang/test/SemaTemplate/gh61159.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 // expected-no-diagnostics
 
-
 namespace GH61159 {
-template<typename T>
-struct X {
-  struct impl;
+template <typename T> struct X {
+  struct I;
+};
+
+template <> struct X<int>::I {
+  template <int ct> constexpr int f() { return ct; };
+
+  int data = 3;
+};
+
+template <typename T> struct X<T>::I {
+  template <T ct> constexpr T f() { return ct + 1; };
+  T data = 7;
 };
 
+static_assert(X<int>::I{}.f<17>() == 17);
+static_assert(X<int>::I{}.data == 3);
+static_assert(X<short>::I{}.data == 7);
+static_assert(X<short>::I{}.f<18>() == 19);
+
+template <typename T> struct Y {
+  struct I;
+};
 
-template <>
-struct X<int>::impl {
-    template<int ct>
-    int f() { return ct; };
+template <> struct Y<int> {
+  struct I {
+    template <int ct> constexpr int f() { return ct; };
+    int data = 3;
   };
+};
+
+static_assert(Y<int>::I{}.f<17>() == 17);
+static_assert(Y<int>::I{}.data == 3);
 
-void foo() {
-    X<int>::impl{}.f<17>();
-}
-}
+} // namespace GH61159