Create a deduction guide for basic_string

[mclow.lists]

Summary

First of the C++17 deduction guides (I think).
Uses the new __is_allocator trait.

The failing test is not quite right yet, but the success bits all work.

[EricWF]

LGTM other than inline comments.

include/string

1491

This should be guarded by _LIBCPP_HAS_NO_DEDUCTION_GUIDES.

[mclow.lists]

This was landed as r324619

include/string

     bool __invariants() const;
 };
 
+template<class InputIterator,
+         class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
+basic_string(InputIterator, InputIterator, Allocator = Allocator())
+   -> basic_string<typename iterator_traits<InputIterator>::value_type,
+                  char_traits<typename iterator_traits<InputIterator>::value_type>,
+                  Allocator>;   // C++17
+
 template<class charT, class traits, class Allocator>
 basic_string<charT, traits, Allocator>
 operator+(const basic_string<charT, traits, Allocator>& lhs,

     friend basic_string operator+<>(const basic_string&, value_type);
 };
 
+#if _LIBCPP_STD_VER > 14
+template<class _InputIterator,
+         class _CharT = typename iterator_traits<_InputIterator>::value_type,
+         class _Allocator = allocator<_CharT>,
+         class = typename enable_if<__is_input_iterator<_InputIterator>::value, void>::type,
+         class = typename enable_if<__is_allocator<_Allocator>::value, void>::type
+         >
+basic_string(_InputIterator, _InputIterator, _Allocator = _Allocator())
+  -> basic_string<_CharT, char_traits<_CharT>, _Allocator>;
+#endif
+
+                  
 template <class _CharT, class _Traits, class _Allocator>
 inline _LIBCPP_INLINE_VISIBILITY
 void

test/std/strings/basic.string/string.cons/iter_alloc.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.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// template<class InputIterator,
+//      class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
+//  basic_string(InputIterator, InputIterator, Allocator = Allocator())
+//    -> basic_string<typename iterator_traits<InputIterator>::value_type,
+//                 char_traits<typename iterator_traits<InputIterator>::value_type>,
+//                 Allocator>;
+//
+//  The deduction guide shall not participate in overload resolution if InputIterator
+//  is a type that does not qualify as an input iterator, or if Allocator is a type
+//  that does not qualify as an allocator.
+
+
+#include <string>
+#include <iterator>
+#include <cassert>
+#include <cstddef>
+
+#include "test_macros.h"
+
+class NotAnItertor {};
+
+template <typename T>
+class NotAnAllocator {};
+
+int main()
+{
+    { // Not an iterator at all
+    std::basic_string s1{NotAnItertor{}, NotAnItertor{}, std::allocator<char>{}}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'basic_string'}}
+    }
+    { // Not an input iterator
+    const char16_t* s = u"12345678901234";
+    std::basic_string<char16_t> s0;
+    std::basic_string s1{std::back_insert_iterator(s0), 
+               std::back_insert_iterator(s0),
+               std::allocator<char16_t>{}};  // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'basic_string'}}
+    }
+    { // Not an allocator
+    const wchar_t* s = L"12345678901234";
+    std::basic_string s1{s, s+10, NotAnAllocator<wchar_t>{}};  // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'basic_string'}}
+    }
+}

test/std/strings/basic.string/string.cons/iter_alloc.pass.cpp

 //   basic_string(InputIterator begin, InputIterator end,
 //   const Allocator& a = Allocator());
 
+// template<class InputIterator,
+//      class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
+//  basic_string(InputIterator, InputIterator, Allocator = Allocator())
+//    -> basic_string<typename iterator_traits<InputIterator>::value_type,
+//                 char_traits<typename iterator_traits<InputIterator>::value_type>,
+//                 Allocator>;
+//
+//  The deduction guide shall not participate in overload resolution if InputIterator
+//  is a type that does not qualify as an input iterator, or if Allocator is a type
+//  that does not qualify as an allocator.
+
+
 #include <string>
 #include <iterator>
 #include <cassert>

     test(input_iterator<const char*>(s), input_iterator<const char*>(s+50), A());
     }
 #endif
+
+//  Test deduction guides
+#if TEST_STD_VER > 14
+    {
+    const char* s = "12345678901234";
+    std::basic_string s1{s, s+10, std::allocator<char>{}};
+    using S = decltype(s1); // what type did we get?
+    static_assert(std::is_same_v<S::value_type,                      char>,  "");
+    static_assert(std::is_same_v<S::traits_type,    std::char_traits<char>>, "");
+    static_assert(std::is_same_v<S::allocator_type,   std::allocator<char>>, "");
+    assert(s1.size() == 10);
+    assert(s1.compare(0, s1.size(), s, s1.size()) == 0);
+    }
+    {
+    const wchar_t* s = L"12345678901234";
+    std::basic_string s1{s, s+10, test_allocator<wchar_t>{}};
+    using S = decltype(s1); // what type did we get?
+    static_assert(std::is_same_v<S::value_type,                      wchar_t>,  "");
+    static_assert(std::is_same_v<S::traits_type,    std::char_traits<wchar_t>>, "");
+    static_assert(std::is_same_v<S::allocator_type,   test_allocator<wchar_t>>, "");
+    assert(s1.size() == 10);
+    assert(s1.compare(0, s1.size(), s, s1.size()) == 0);
+    }
+    {
+    const char16_t* s = u"12345678901234";
+    std::basic_string s1{s, s+10, min_allocator<char16_t>{}};
+    using S = decltype(s1); // what type did we get?
+    static_assert(std::is_same_v<S::value_type,                      char16_t>,  "");
+    static_assert(std::is_same_v<S::traits_type,    std::char_traits<char16_t>>, "");
+    static_assert(std::is_same_v<S::allocator_type,    min_allocator<char16_t>>, "");
+    assert(s1.size() == 10);
+    assert(s1.compare(0, s1.size(), s, s1.size()) == 0);
+    }
+    {
+    const char32_t* s = U"12345678901234";
+    std::basic_string s1{s, s+10, explicit_allocator<char32_t>{}};
+    using S = decltype(s1); // what type did we get?
+    static_assert(std::is_same_v<S::value_type,                        char32_t>,  "");
+    static_assert(std::is_same_v<S::traits_type,      std::char_traits<char32_t>>, "");
+    static_assert(std::is_same_v<S::allocator_type, explicit_allocator<char32_t>>, "");
+    assert(s1.size() == 10);
+    assert(s1.compare(0, s1.size(), s, s1.size()) == 0);
+    }
+#endif
 }