P1989R2 Range Constructor For `string_view` (#2000)

Co-authored-by: timsong-cpp <rs2740@gmail.com>
Co-authored-by: Casey Carter <cartec69@gmail.com>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/xstring

@@ -1289,6 +1289,25 @@ public:
     constexpr basic_string_view(_It _First, _Se _Last) noexcept(noexcept(_Last - _First)) // strengthened
         : _Mydata(_STD to_address(_First)), _Mysize(static_cast<size_type>(_Last - _First)) {}
     // clang-format on
+
+#if _HAS_CXX23
+    // clang-format off
+    template <_RANGES contiguous_range _Range>
+        requires _RANGES sized_range<_Range>
+            && same_as<_RANGES range_value_t<_Range>, _Elem>
+            && (!is_convertible_v<_Range, const _Elem*>)
+            && (!requires(remove_cvref_t<_Range>& _Rng) {
+                _Rng.operator _STD basic_string_view<_Elem, _Traits>();
+            })
+            && (!requires {
+                // per editorial or LWG issue not yet filed as of 2021-06-10
+                typename remove_reference_t<_Range>::traits_type;
+            } || same_as<typename remove_reference_t<_Range>::traits_type, _Traits>)
+    constexpr basic_string_view(_Range&& _Rng) noexcept(
+        noexcept(_RANGES data(_Rng)) && noexcept(_RANGES size(_Rng))) // strengthened
+        : _Mydata(_RANGES data(_Rng)), _Mysize(static_cast<size_t>(_RANGES size(_Rng))) {}
+    // clang-format on
+#endif // _HAS_CXX23
 #endif // __cpp_lib_concepts
 
     _NODISCARD constexpr const_iterator begin() const noexcept {
@@ -1696,6 +1715,11 @@ private:
 template <contiguous_iterator _It, sized_sentinel_for<_It> _Se>
 basic_string_view(_It, _Se) -> basic_string_view<iter_value_t<_It>>;
 
+#if _HAS_CXX23
+template <_RANGES contiguous_range _Range>
+basic_string_view(_Range&&) -> basic_string_view<_RANGES range_value_t<_Range>>;
+#endif // _HAS_CXX23
+
 namespace ranges {
     template <class _Elem, class _Traits>
     inline constexpr bool enable_view<basic_string_view<_Elem, _Traits>> = true;

stl/inc/yvals_core.h

@@ -263,6 +263,7 @@
 // P1048R1 is_scoped_enum
 // P1679R3 contains() For basic_string/basic_string_view
 // P1682R3 to_underlying() For Enumerations
+// P1989R2 Range Constructor For string_view
 // P2166R1 Prohibiting basic_string And basic_string_view Construction From nullptr
 
 // Parallel Algorithms Notes

tests/std/tests/P0220R1_string_view/test.cpp

@@ -3,12 +3,14 @@
 
 #include <array>
 #include <assert.h>
+#include <deque>
 #include <sstream>
 #include <stdexcept>
 #include <stdlib.h>
 #include <string>
 #include <string_view>
 #include <type_traits>
+#include <vector>
 
 #include <constexpr_char_traits.hpp>
 
@@ -328,6 +330,31 @@ constexpr bool test_case_contiguous_constructor() {
     return true;
 }
 
+constexpr bool test_case_range_constructor() {
+#if _HAS_CXX23 && defined(__cpp_lib_concepts)
+    const array expectedData{'n', 'o', ' ', 'n', 'u', 'l', 'l'};
+    // Also tests the corresponding deduction guide:
+    same_as<string_view> auto sv = basic_string_view(expectedData);
+    assert(sv.data() == expectedData.data());
+    assert(sv.size() == 7);
+
+    // Also tests some of the constraints:
+    static_assert(is_constructible_v<string_view, vector<char>>);
+    static_assert(is_convertible_v<vector<char>, string_view>);
+
+    static_assert(!is_constructible_v<string_view, deque<char>>); // not contiguous
+    static_assert(!is_convertible_v<deque<char>, string_view>);
+
+    static_assert(!is_constructible_v<string_view, vector<unsigned char>>); // different elements
+    static_assert(!is_convertible_v<vector<unsigned char>, string_view>);
+
+    static_assert(!is_constructible_v<string_view, basic_string<char, constexpr_char_traits>>); // different traits
+    static_assert(!is_convertible_v<basic_string<char, constexpr_char_traits>, string_view>);
+#endif // _HAS_CXX23 && defined(__cpp_lib_concepts)
+
+    return true;
+}
+
 template <class CharT, class Traits>
 constexpr bool test_case_iterators() {
     using iterator = typename basic_string_view<CharT, Traits>::iterator;
@@ -1162,6 +1189,7 @@ static_assert(test_case_default_constructor());
 static_assert(test_case_ntcts_constructor<constexpr_char_traits>());
 static_assert(test_case_buffer_constructor());
 static_assert(test_case_contiguous_constructor());
+static_assert(test_case_range_constructor());
 #if defined(__clang__) || defined(__EDG__) // TRANSITION, VSO-284079 "C1XX's C++14 constexpr emits bogus warnings C4146,
                                            // C4308, C4307 for basic_string_view::iterator"
 static_assert(test_case_iterators<char, constexpr_char_traits>());
@@ -1217,6 +1245,7 @@ int main() {
     test_case_ntcts_constructor();
     test_case_buffer_constructor();
     test_case_contiguous_constructor();
+    test_case_range_constructor();
     test_case_iterators<char, char_traits<char>>();
     test_case_iterators<wchar_t, char_traits<wchar_t>>();
     test_case_prefix<char, char_traits<char>>();