Implement LWG#2778: basic_string_view is missing constexpr.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292823 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/string.view/string.view.cons/assign.pass.cpp

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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_view>
+
+// constexpr basic_string_view& operator=(const basic_string_view &) noexcept = default;
+
+#include <string_view>
+#include <cassert>
+
+#include "test_macros.h"
+
+template<typename T>
+#if TEST_STD_VER > 11
+constexpr
+#endif
+bool test (T sv0)
+    {
+    T sv1;
+    sv1 = sv0;
+//  We can't just say "sv0 == sv1" here because string_view::compare
+//  isn't constexpr until C++17, and we want to support back to C++14
+    return sv0.size() == sv1.size() && sv0.data() == sv1.data();
+    }
+
+int main () {
+
+    assert( test<std::string_view>    ( "1234"));
+#ifdef _LIBCPP_HAS_NO_UNICODE_CHARS
+    assert( test<std::u16string_view> (u"1234"));
+    assert( test<std::u32string_view> (U"1234"));
+#endif
+    assert( test<std::wstring_view>   (L"1234"));
+
+#if TEST_STD_VER > 11
+    static_assert( test<std::string_view>    ({ "abc", 3}), "");
+#ifdef _LIBCPP_HAS_NO_UNICODE_CHARS
+    static_assert( test<std::u16string_view> ({u"abc", 3}), "");
+    static_assert( test<std::u32string_view> ({U"abc", 3}), "");
+#endif
+    static_assert( test<std::wstring_view>   ({L"abc", 3}), "");
+#endif
+}