[libcxx] Fix LWG Issue #2367 - Fixing std::tuple and std::pair's default constructors.

[EricWF]

Summary

This patch implements the solution for LWG Issue #2367. See http://cplusplus.github.io/LWG/lwg-active.html#2367

[EricWF]

Fix merge conflicts.

[EricWF]

Address some of @mclow.lists offline comments. I cleaned up the tests.

[EricWF]

Accepting. Any additional review can happen post-commit.

include/__config

 #if __has_feature(cxx_attributes)
 #  define _LIBCPP_NORETURN [[noreturn]]
 #else
 #  define _LIBCPP_NORETURN __attribute__ ((noreturn))
 #endif
 
 #define _LIBCPP_UNUSED __attribute__((__unused__))
 
+#if !(__has_feature(cxx_default_function_template_args))
+#define _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
+#endif
+
 #if !(__has_feature(cxx_defaulted_functions))
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #endif  // !(__has_feature(cxx_defaulted_functions))
 
 #if !(__has_feature(cxx_deleted_functions))
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #endif  // !(__has_feature(cxx_deleted_functions))
 

 #if !defined(__cpp_variable_templates) || __cpp_variable_templates < 201304L
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 
 #ifndef __GXX_EXPERIMENTAL_CXX0X__
 
 #define _LIBCPP_HAS_NO_ADVANCED_SFINAE
 #define _LIBCPP_HAS_NO_DECLTYPE
+#define _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #define _LIBCPP_HAS_NO_NULLPTR
 #define _LIBCPP_HAS_NO_STATIC_ASSERT
 #define _LIBCPP_HAS_NO_UNICODE_CHARS
 #define _LIBCPP_HAS_NO_VARIADICS
 #define _LIBCPP_HAS_NO_RVALUE_REFERENCES
 #define _LIBCPP_HAS_NO_STRONG_ENUMS
 #define _LIBCPP_HAS_NO_NOEXCEPT
 
 #else  // __GXX_EXPERIMENTAL_CXX0X__
 
 #if _GNUC_VER < 403
+#define _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
 #define _LIBCPP_HAS_NO_RVALUE_REFERENCES
-#endif
-
-#if _GNUC_VER < 403
 #define _LIBCPP_HAS_NO_STATIC_ASSERT
 #endif
 
+
 #if _GNUC_VER < 404
 #define _LIBCPP_HAS_NO_DECLTYPE
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #define _LIBCPP_HAS_NO_TRAILING_RETURN
 #define _LIBCPP_HAS_NO_UNICODE_CHARS
 #define _LIBCPP_HAS_NO_VARIADICS
 #define _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
 #endif  // _GNUC_VER < 404

 #elif defined(__IBMCPP__)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 #define _ATTRIBUTE(x) __attribute__((x))
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 #define _LIBCPP_UNUSED
 
+#define _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
 #define _LIBCPP_HAS_NO_TEMPLATE_ALIASES
 #define _LIBCPP_HAS_NO_ADVANCED_SFINAE
 #define _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
 #define _LIBCPP_HAS_NO_NOEXCEPT
 #define _LIBCPP_HAS_NO_NULLPTR
 #define _LIBCPP_HAS_NO_UNICODE_CHARS
 #define _LIBCPP_HAS_IS_BASE_OF
 #define _LIBCPP_HAS_IS_FINAL

include/utility

struct _LIBCPP_TYPE_VIS_ONLY pair
     typedef _T2 second_type;
 
     _T1 first;
     _T2 second;
 
     // pair(const pair&) = default;
     // pair(pair&&) = default;
 
+#ifndef _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
+    template <bool _Dummy = true, class = typename enable_if<
+        __dependent_type<is_default_constructible<_T1>, _Dummy>::value &&
+        __dependent_type<is_default_constructible<_T2>, _Dummy>::value
+      >::type>
+#endif
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR pair() : first(), second() {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(const _T1& __x, const _T2& __y)
         : first(__x), second(__y) {}
 
     template<class _U1, class _U2>
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11

test/std/utilities/utility/pairs/pairs.pair/default.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.
 //
 //===----------------------------------------------------------------------===//
 
 // <utility>
 
 // template <class T1, class T2> struct pair
 
 // constexpr pair();
 
 #include <utility>
+#include <type_traits>
 #include <cassert>
 
+#include "test_macros.h"
+
+
+#if TEST_STD_VER > 11
+#define CONSTEXPR_CXX14 constexpr
+    // Expands to 'constexpr' in C++14 and greater and '' otherwise.
+#define STATIC_ASSERT_CXX14(Pred) static_assert(Pred, "")
+    // Assert the specified expression 'Pred' using a static_assert in C++14 and
+    // greater and using assert(...) otherwise.
+#else
+#define CONSTEXPR_CXX14
+#define STATIC_ASSERT_CXX14(Pred) assert(Pred)
+#endif
+
+#if TEST_STD_VER >= 11
+#define ASSERT_CXX03_ONLY(Pred) ((void)0)
+    // Assert the specified expression 'Pred' only in C++98 and C++03. In C++11
+    // and greater 'ASSERT_CXX03_ONLY' expands to a nop expression.
+#else
+#define ASSERT_CXX03_ONLY(Pred) assert(Pred)
+#endif
+
+
+#if TEST_STD_VER >= 11
+struct DeletedDefault {
+    // A class with a deleted default constructor. Used to test the SFINAE
+    // on std::pairs default constructor.
+    constexpr explicit DeletedDefault(int x) : value(x) {}
+    constexpr DeletedDefault() = delete;
+    int value;
+};
+#endif
+
+template <class T>
+struct IllFormedDefaultImp {
+  TEST_CONSTEXPR explicit IllFormedDefaultImp(int v) : value(v) {}
+  TEST_CONSTEXPR IllFormedDefaultImp() : value(T::DoesNotExistAndShouldNotCompile) {}
+  int value;
+};
+
+typedef IllFormedDefaultImp<int> IllFormedDefault;
+    // A class which provides a constexpr default constructor with a valid
+    // signature but an ill-formed body. The A compile error will be emitted if
+    // the default constructor is instantiated.
+
+
+// Test that the defaulted constructor does not participate in overload
+// resolution when either of the types in the pair is not default constructible.
+// See LWG issue #2367
+void test_deleted_default()
+{
+#if TEST_STD_VER >= 11
+    {
+    typedef std::pair<int, int> P;
+    static_assert(std::is_default_constructible<P>::value, "");
+    static_assert(std::is_constructible<P>::value, "");
+    }
+    {
+    typedef std::pair<DeletedDefault, int> P;
+    static_assert(!std::is_default_constructible<P>::value, "");
+    static_assert(!std::is_constructible<P>::value, "");
+    static_assert(std::is_constructible<P, DeletedDefault, int>::value, "");
+    CONSTEXPR_CXX14 P p(DeletedDefault(42), -5);
+    STATIC_ASSERT_CXX14(p.first.value == 42 && p.second == -5);
+    }
+    {
+    typedef std::pair<int, DeletedDefault> P;
+    static_assert(!std::is_default_constructible<P>::value, "");
+    static_assert(!std::is_constructible<P>::value, "");
+    static_assert(std::is_constructible<P, int, DeletedDefault>::value, "");
+    constexpr DeletedDefault dd(-5);
+    CONSTEXPR_CXX14 P p(42, dd);
+    STATIC_ASSERT_CXX14(p.first == 42 && p.second.value == -5);
+    }
+    {
+    typedef std::pair<DeletedDefault, DeletedDefault> P;
+    static_assert(!std::is_default_constructible<P>::value, "");
+    static_assert(std::is_constructible<P, DeletedDefault, DeletedDefault>::value, "");
+    CONSTEXPR_CXX14 P p(DeletedDefault(42), DeletedDefault(-5));
+    STATIC_ASSERT_CXX14(p.first.value == 42 && p.second.value == -5);
+    }
+#endif
+}
+
+
+// Check that the SFINAE on the default constructor is not evaluated when
+// it isn't needed. If the default constructor of 'IllFormedDefault' is evaluated
+// in C++11, even with is_default_constructible, then this test should fail to
+// compile. In C++14 and greater evaluate each test is evaluated as a constant
+// expression. In C++03 evaluating is_default_constructible will compile return the
+// wrong answer because libc++ does not back-port this behavior.
+// See LWG issue #2367
+void test_illformed_default()
+{
+    {
+    typedef std::pair<IllFormedDefault, int> P;
+    static_assert((std::is_constructible<P, IllFormedDefault, int>::value), "");
+    CONSTEXPR_CXX14 P p(IllFormedDefault(42), -5);
+    STATIC_ASSERT_CXX14(p.first.value == 42 && p.second == -5);
+    ASSERT_CXX03_ONLY(std::is_default_constructible<P>::value); // ASSERTS BUG IN C++03
+    }
+    {
+    typedef std::pair<int, IllFormedDefault> P;
+    static_assert((std::is_constructible<P, int, IllFormedDefault>::value), "");
+    CONSTEXPR_CXX14 IllFormedDefault dd(-5);
+    CONSTEXPR_CXX14 P p(42, dd);
+    STATIC_ASSERT_CXX14(p.first == 42 && p.second.value == -5);
+    ASSERT_CXX03_ONLY(std::is_default_constructible<P>::value); // ASSERTS BUG IN C++03
+    }
+    {
+    typedef std::pair<IllFormedDefault, IllFormedDefault> P;
+    static_assert((std::is_constructible<P, IllFormedDefault, IllFormedDefault>::value), "");
+    CONSTEXPR_CXX14 P p(IllFormedDefault(42), IllFormedDefault(-5));
+    STATIC_ASSERT_CXX14(p.first.value == 42 && p.second.value == -5);
+    ASSERT_CXX03_ONLY(std::is_default_constructible<P>::value); // ASSERTS BUG IN C++03
+    }
+}
+
+
 int main()
 {
     {
     typedef std::pair<float, short*> P;
     P p;
     assert(p.first == 0.0f);
     assert(p.second == nullptr);
     }
-    
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER >= 11
     {
     typedef std::pair<float, short*> P;
     constexpr P p;
     static_assert(p.first == 0.0f, "");
     static_assert(p.second == nullptr, "");
     }
 #endif
+    test_deleted_default();
+    test_illformed_default();
 }