[libcxx] Re-implement LWG 2770 again: Fix tuple_size to work with structured bindings

[EricWF]

Summary

This patch attempts to re-implement a fix for LWG 2770, but not the actual specified PR.

The PR for 2770 specifies tuple_size<T const> as only conditionally providing a ::value member. However C++17 structured bindings require tuple_size<T const> to be complete only if tuple_size<T> is also complete. Therefore this patch implements only provides the specialization tuple_size<T CV> iff tuple_size<T> is a complete type.

This fixes http://llvm.org/PR31513.

[EricWF]

Use the correct patch file this time.

[rsmith]

LGTM, but a minor simplification seems possible.

include/__tuple

32

remove_cv looks redundant here, deduction already stripped the cv qualifiers.

[EricWF]

include/__tuple

32

The remove_cv should be redundant, but without it Clang segfaults in clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformTemplateSpecializationType(clang::TypeLocBuilder&, clang::TemplateSpecializationTypeLoc, clang::TemplateName).

I haven't had time to file a Clang bug yet.

[EricWF]

I thought this formulation worked for GCC and Clang but it appears it does not. I'll update with another attempt.

[EricWF]

@rsmith Could you re-review this please? It's a mostly different implementation.

[EricWF]

• Remove redundant SFINAE.

[EricWF]

• SFINAE the specializations on sizeof(tuple_size<T>) instead of decltype(tuple_size<T>::value).

[EricWF]

include/__tuple

29

Without this extra enable_if, GCC will diagnose each of these three specializations as ambiguous.

[mpark]

That was fun =D

[EricWF]

Updating with final changes.

include/__tuple

 #pragma GCC system_header
 #endif
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp> class _LIBCPP_TYPE_VIS_ONLY tuple_size;
 
-struct __empty_tuple_size_base {};
-
-template <class _Tp, class = void>
-struct __tuple_size_base_type {
-  typedef __empty_tuple_size_base type;
-};
-
-template <class _Tp>
-struct __tuple_size_base_type<_Tp, typename __void_t<decltype(tuple_size<_Tp>::value)>::type>
-{
-  typedef integral_constant<size_t, tuple_size<_Tp>::value> type;
-};
+#ifndef _LIBCPP_CXX03_LANG
+template <class _Tp, class>
+using __enable_if_tuple_size_imp = _Tp;
 
 template <class _Tp>
-class _LIBCPP_TYPE_VIS_ONLY tuple_size<const _Tp>
-    : public __tuple_size_base_type<_Tp>::type {};
+class _LIBCPP_TYPE_VIS_ONLY tuple_size<__enable_if_tuple_size_imp<const _Tp, decltype(tuple_size<typename remove_cv<_Tp>::type>::value)>>
+    : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TYPE_VIS_ONLY tuple_size<volatile _Tp>
-    : public __tuple_size_base_type<_Tp>::type {};
+class _LIBCPP_TYPE_VIS_ONLY tuple_size<__enable_if_tuple_size_imp<volatile _Tp, decltype(tuple_size<typename remove_cv<_Tp>::type>::value)>>
+    : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TYPE_VIS_ONLY tuple_size<const volatile _Tp>
-    : public __tuple_size_base_type<_Tp>::type {};
+class _LIBCPP_TYPE_VIS_ONLY tuple_size<__enable_if_tuple_size_imp<const volatile _Tp, decltype(tuple_size<typename remove_cv<_Tp>::type>::value)>>
+    : public integral_constant<size_t, tuple_size<_Tp>::value> {};
+#else
+template <class _Tp> class _LIBCPP_TYPE_VIS_ONLY tuple_size<const _Tp> : tuple_size<_Tp> {};
+template <class _Tp> class _LIBCPP_TYPE_VIS_ONLY tuple_size<volatile _Tp> : tuple_size<_Tp> {};
+template <class _Tp> class _LIBCPP_TYPE_VIS_ONLY tuple_size<const volatile _Tp> : tuple_size<_Tp> {};
+#endif
 
 template <size_t _Ip, class _Tp> class _LIBCPP_TYPE_VIS_ONLY tuple_element;
 
 template <size_t _Ip, class _Tp>
 class _LIBCPP_TYPE_VIS_ONLY tuple_element<_Ip, const _Tp>
 {
 public:
     typedef typename add_const<typename tuple_element<_Ip, _Tp>::type>::type type;

test/libcxx/test/config.py

def configure_features(self):
         else:
             # FIXME remove this once more than just clang-4.0 support
             # C++17 aligned allocation.
             self.config.available_features.add('no-aligned-allocation')
 
         if self.get_lit_bool('has_libatomic', False):
             self.config.available_features.add('libatomic')
 
-        if '__cpp_if_constexpr' not in self.cxx.dumpMacros():
+        macros = self.cxx.dumpMacros()
+        if '__cpp_if_constexpr' not in macros:
             self.config.available_features.add('libcpp-no-if-constexpr')
 
+        if '__cpp_structured_bindings' not in macros:
+            self.config.available_features.add('libcpp-no-structured-bindings')
+
     def configure_compile_flags(self):
         no_default_flags = self.get_lit_bool('no_default_flags', False)
         if not no_default_flags:
             self.configure_default_compile_flags()
         # This include is always needed so add so add it regardless of
         # 'no_default_flags'.
         support_path = os.path.join(self.libcxx_src_root, 'test/support')
         self.cxx.compile_flags += ['-I' + support_path]

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class... Types>
+//   class tuple_size<tuple<Types...>>
+//     : public integral_constant<size_t, sizeof...(Types)> { };
+
+// UNSUPPORTED: c++98, c++03
+
+#include <tuple>
+#include <array>
+#include <type_traits>
+
+template <class T, size_t Size = sizeof(std::tuple_size<T>)>
+constexpr bool is_complete(int) { static_assert(Size > 0, ""); return true; }
+template <class> constexpr bool is_complete(long) { return false; }
+template <class T> constexpr bool is_complete() { return is_complete<T>(0); }
+
+struct Dummy1 {};
+struct Dummy2 {};
+struct Dummy3 {};
+
+template <>
+class std::tuple_size<Dummy1> {
+public:
+  static size_t value;
+};
+
+template <>
+class std::tuple_size<Dummy2> {
+public:
+  static void value() {}
+};
+
+template <>
+class std::tuple_size<Dummy3> {};
+
+template <class T>
+void test_complete() {
+  static_assert(is_complete<T>(), "");
+  static_assert(is_complete<const T>(), "");
+  static_assert(is_complete<volatile T>(), "");
+  static_assert(is_complete<const volatile T>(), "");
+}
+
+template <class T>
+void test_incomplete() {
+  static_assert(!is_complete<T>(), "");
+  static_assert(!is_complete<const T>(), "");
+  static_assert(!is_complete<volatile T>(), "");
+  static_assert(!is_complete<const volatile T>(), "");
+}
+
+int main()
+{
+  // Test that tuple_size<const T> is not incomplete when tuple_size<T>::value
+  // is well-formed but not a constant expression.
+  {
+    // expected-error@__tuple:* 1 {{is not a constant expression}}
+    (void)std::tuple_size<const Dummy1>::value; // expected-note {{here}}
+  }
+  // Test that tuple_size<const T> is not incomplete when tuple_size<T>::value
+  // is well-formed but not convertible to size_t.
+  {
+    // expected-error@__tuple:* 1 {{value of type 'void ()' is not implicitly convertible to 'unsigned long'}}
+    (void)std::tuple_size<const Dummy2>::value; // expected-note {{here}}
+  }
+  // Test that tuple_size<const T> is incomplete when tuple_size<T>::value is
+  // ill-formed.
+  // FIXME: it would be better if tuple_size<const Dummy3> were a complete type
+  // which generated diagnostics when used.
+  {
+    (void)std::tuple_size<const Dummy3>::value;
+    // expected-error@-1 {{implicit instantiation of undefined template}}
+  }
+}

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class... Types>
+//   class tuple_size<tuple<Types...>>
+//     : public integral_constant<size_t, sizeof...(Types)> { };
+
+// UNSUPPORTED: c++98, c++03
+
+#include <tuple>
+#include <array>
+#include <type_traits>
+
+template <class T, size_t Size = sizeof(std::tuple_size<T>)>
+constexpr bool is_complete(int) { static_assert(Size > 0, ""); return true; }
+template <class> constexpr bool is_complete(long) { return false; }
+template <class T> constexpr bool is_complete() { return is_complete<T>(0); }
+
+struct Dummy1 {};
+struct Dummy2 {};
+
+template <>
+class std::tuple_size<Dummy1> : public std::integral_constant<size_t, 0> {};
+
+template <class T>
+void test_complete() {
+  static_assert(is_complete<T>(), "");
+  static_assert(is_complete<const T>(), "");
+  static_assert(is_complete<volatile T>(), "");
+  static_assert(is_complete<const volatile T>(), "");
+}
+
+template <class T>
+void test_incomplete() {
+  static_assert(!is_complete<T>(), "");
+  static_assert(!is_complete<const T>(), "");
+  static_assert(!is_complete<volatile T>(), "");
+  static_assert(!is_complete<const volatile T>(), "");
+}
+
+
+int main()
+{
+  test_complete<std::tuple<> >();
+  test_complete<std::tuple<int&> >();
+  test_complete<std::tuple<int&&, int&, void*>>();
+  test_complete<std::pair<int, long> >();
+  test_complete<std::array<int, 5> >();
+  test_complete<Dummy1>();
+
+  test_incomplete<void>();
+  test_incomplete<int>();
+  test_incomplete<std::tuple<int>&>();
+  test_incomplete<Dummy2>();
+}

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class... Types>
+//   class tuple_size<tuple<Types...>>
+//     : public integral_constant<size_t, sizeof...(Types)> { };
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// XFAIL: libcpp-no-structured-bindings
+
+#include <tuple>
+#include <array>
+#include <type_traits>
+#include <cassert>
+
+struct S { int x; };
+
+void test_decomp_user_type() {
+  {
+    S s{99};
+    auto [m1] = s;
+    auto& [r1] = s;
+    assert(m1 == 99);
+    assert(&r1 == &s.x);
+  }
+  {
+    S const s{99};
+    auto [m1] = s;
+    auto& [r1] = s;
+    assert(m1 == 99);
+    assert(&r1 == &s.x);
+  }
+}
+
+void test_decomp_tuple() {
+  typedef std::tuple<int> T;
+  {
+    T s{99};
+    auto [m1] = s;
+    auto& [r1] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+  {
+    T const s{99};
+    auto [m1] = s;
+    auto& [r1] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+}
+
+
+void test_decomp_pair() {
+  typedef std::pair<int, double> T;
+  {
+    T s{99, 42.1};
+    auto [m1, m2] = s;
+    auto& [r1, r2] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+  {
+    T const s{99, 42.1};
+    auto [m1, m2] = s;
+    auto& [r1, r2] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+}
+
+void test_decomp_array() {
+  typedef std::array<int, 3> T;
+  {
+    T s{{99, 42, -1}};
+    auto [m1, m2, m3] = s;
+    auto& [r1, r2, r3] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+  {
+    T const s{{99, 42, -1}};
+    auto [m1, m2, m3] = s;
+    auto& [r1, r2, r3] = s;
+    assert(m1 == 99);
+    assert(&r1 == &std::get<0>(s));
+  }
+}
+
+int main() {
+  test_decomp_user_type();
+  test_decomp_tuple();
+  test_decomp_pair();
+  test_decomp_array();
+}