[clang] Add test for CWG203 "Type of address-of-member expression"

[Endilll]

This patch adds test for CWG203. Author was asking to change the type of pointer-to-member expression to be closer to how it's written as opposed to where the resulting member belongs to, but was turned down due to backwards compatibility concerns, so we're testing the status quo.

There are a total of 6 examples in the filing, so I decided to just throw all of them into the test. I had to turn example 2 into constexpr test that unfortunately requires C++20. Outcomes in example 5 that Tomasz expected are not in line with implementation behavior and my reading of the Standard. I think he got confused by the fact that unlike regular pointers, pointers-to-members can be implicitly downcasted, but not upcasted. I left comments in the example.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

Changes

This patch adds test for CWG203. Author was asking to change the type of pointer-to-member expression to be closer to how it's written as opposed to where the resulting member belongs to, but was turned down due to backwards compatibility concerns, so we're testing the status quo.

There are a total of 6 examples in the filing, so I decided to just throw all of them into the test. I had to turn example 2 into constexpr test that unfortunately requires C++20. Outcomes in example 5 that Tomasz expected are not in line with implementation behavior and my reading of the Standard. I think he got confused by the fact that unlike regular pointers, pointers-to-members can be implicitly downcasted, but not upcasted. I left comments in the example.

---

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

2 Files Affected:

• (modified) clang/test/CXX/drs/cwg2xx.cpp (+136-2)
• (modified) clang/www/cxx_dr_status.html (+1-1)

diff --git a/clang/test/CXX/drs/cwg2xx.cpp b/clang/test/CXX/drs/cwg2xx.cpp
index ec37b420880e28..6a1dfcf2774779 100644
--- a/clang/test/CXX/drs/cwg2xx.cpp
+++ b/clang/test/CXX/drs/cwg2xx.cpp
@@ -2,8 +2,9 @@
 // RUN: %clang_cc1 -std=c++11 %s -verify=expected,since-cxx11,cxx98-11,cxx98-14,cxx98-17 -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 %s -verify=expected,since-cxx11,since-cxx14,cxx98-14,cxx98-17 -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,cxx98-17 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++20 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++23 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++20 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,since-cxx20 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++23 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,since-cxx20 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2c %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,since-cxx20 -fexceptions -fcxx-exceptions -pedantic-errors
 
 // FIXME: diagnostic above is emitted only on Windows platforms
 // PR13819 -- __SIZE_TYPE__ is incompatible.
@@ -41,6 +42,139 @@ namespace cwg202 { // cwg202: 3.1
   template struct X<f>;
 }
 
+namespace cwg203 { // cwg203: 2.8
+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@-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, "");
+} // namespace ex2
+#endif
+
+namespace ex3 {
+class Base {
+public:
+  int func() const;
+};
+
+class Derived : public Base {};
+
+template <class T> class Templ { // #cwg203-ex3-Templ
+public:
+  template <class S> Templ(S (T::*ptmf)() const); // #cwg203-ex3-Templ-ctor
+};
+
+void foo() { Templ<Derived> x(&Derived::func); }
+// expected-error@-1 {{<source>:46:29: error: no matching constructor for initialization of 'Templ<Derived>'}}
+//   expected-note@#cwg203-ex3-Templ {{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int (cwg203::ex3::Base::*)() const' to 'const Templ<Derived>' for 1st argument}}
+//   expected-note@#cwg203-ex3-Templ {{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int (cwg203::ex3::Base::*)() const' to 'Templ<Derived>' for 1st argument}}
+//   expected-note@#cwg203-ex3-Templ-ctor {{candidate template ignored: could not match 'cwg203::ex3::Derived' against 'cwg203::ex3::Base'}}
+} // namespace ex3
+
+namespace ex4 {
+struct Very_base {
+  int a;
+};
+struct Base1 : Very_base {};
+struct Base2 : Very_base {};
+struct Derived : Base1, Base2 {
+};
+
+int f() {
+  Derived d;
+  int Derived::*a_ptr = &Derived::Base1::a;
+  /* expected-error@-1
+  {{ambiguous conversion from pointer to member of base class 'cwg203::ex4::Very_base' to pointer to member of derived class 'cwg203::ex4::Derived':
+    struct cwg203::ex4::Derived -> Base1 -> Very_base
+    struct cwg203::ex4::Derived -> Base2 -> Very_base}}*/
+};
+} // namespace ex4
+
+namespace ex5 {
+struct Base {
+  int a;
+};
+struct Derived : Base {
+  int b;
+};
+
+template <typename Class, typename Member_type, Member_type Base::*ptr>
+Member_type get(Class &c) {
+  return c.*ptr;
+}
+
+void call(int (*f)(Derived &)); // #cwg203-ex5-call
+
+int main() {
+  // ill-formed, contrary to Core issue filing:
+  // `&Derived::b` yields `int Derived::*`, which can't initialize NTTP of type `int Base::*`,
+  // because (implicit) pointer-to-member conversion doesn't upcast.
+  call(&get<Derived, int, &Derived::b>);
+  // expected-error@-1 {{no matching function for call to 'call'}}
+  //   expected-note@#cwg203-ex5-call {{candidate function not viable: no overload of 'get' matching 'int (*)(Derived &)' for 1st argument}}
+
+  // well-formed, contrary to Core issue filing:
+  // `&Derived::a` yields `int Base::*`,
+  // which can initialize NTTP of type `int Base::*`.
+  call(&get<Derived, int, &Derived::a>);
+
+  call(&get<Base, int, &Derived::a>);
+  // expected-error@-1 {{no matching function for call to 'call'}}
+  //   expected-note@#cwg203-ex5-call {{candidate function not viable: no overload of 'get' matching 'int (*)(Derived &)' for 1st argument}}
+}
+} // namespace ex5
+
+namespace ex6 {
+struct Base {
+  int a;
+};
+struct Derived : private Base { // #cwg203-ex6-Derived
+public:
+  using Base::a; // make `a` accessible
+};
+
+int main() {
+  Derived d;
+  int b = d.a;
+  int Derived::*ptr = &Derived::a;
+  // expected-error@-1 {{cannot cast private base class 'cwg203::ex6::Base' to 'cwg203::ex6::Derived'}}
+  //   expected-note@#cwg203-ex6-Derived {{declared private here}}
+}
+} // namespace ex6
+} // namespace cwg203
+
 // cwg204: sup 820
 
 namespace cwg206 { // cwg206: yes
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index c069e155fd547c..a383803df670c1 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -1263,7 +1263,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/203.html">203</a></td>
     <td>NAD</td>
     <td>Type of address-of-member expression</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 2.8</td>
   </tr>
   <tr id="204">
     <td><a href="https://cplusplus.github.io/CWG/issues/204.html">204</a></td>

[cor3ntin]

LGTM modulo comments

But I wonder how much we want to focus on testing these NAD core issues that are merely feature requests

[cor3ntin]

Suggested change

	static_assert(DefaultValue(&C::j) == 5, "");
	static_assert(DefaultValue(&C::j) == 5);

[Endilll]

There are only 10 cases of C++17-style static_assert through C++ DR tests, so I'd like to remain consistent.