[clang] [clang] Add test for CWG203 "Type of address-of-member expression" (PR #121687)

[via cfe-commits]

================
@@ -41,6 +42,141 @@ namespace cwg202 { // cwg202: 3.1
   template struct X<f>;
 }
 
+namespace cwg203 { // cwg203: 3.0
+namespace ex1 {
+struct B {
+  int i;
+};
+struct D1 : B {};
+struct D2 : B {};
+
+int(D1::*pmD1) = &D2::i;
+} // namespace ex1
+
+#if __cplusplus >= 202002L
+namespace ex2 {
+struct A {
+  int i;
+  virtual void f() = 0; // #cwg203-ex2-A-f
+};
+
+struct B : A {
+  int j;
+  constexpr B() : j(5) {}
+  virtual void f();
+};
+
+struct C : B {
+  constexpr C() { j = 10; }
+};
+
+template <class T>
+constexpr int DefaultValue(int(T::*m)) {
+  return T().*m;
+  // since-cxx20-error at -1 {{allocating an object of abstract class type 'cwg203::ex2::A'}}
+  //   since-cxx20-note@#cwg203-ex2-a {{in instantiation of function template specialization 'cwg203::ex2::DefaultValue<cwg203::ex2::A>' requested here}}
+  //   since-cxx20-note@#cwg203-ex2-A-f {{unimplemented pure virtual method 'f' in 'A'}}
+} // #cwg203-ex2-DefaultValue
+
+int a = DefaultValue(&B::i); // #cwg203-ex2-a
+static_assert(DefaultValue(&C::j) == 5, "");
----------------
cor3ntin wrote:

```suggestion
static_assert(DefaultValue(&C::j) == 5);
```

https://github.com/llvm/llvm-project/pull/121687