Implement LWG#2761: 'basic_string should require that charT match traits::char_type'. Tests for string_view, too

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@297872 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string_view b/include/string_view
index 8be6132..5c42b36 100644
--- a/include/string_view
+++ b/include/string_view
@@ -199,6 +199,10 @@
 	typedef ptrdiff_t                                  difference_type;
 	static _LIBCPP_CONSTEXPR const size_type npos = -1; // size_type(-1);
 
+    static_assert(is_pod<value_type>::value, "Character type of basic_string_view must be a POD");
+    static_assert((is_same<_CharT, typename traits_type::char_type>::value),
+                  "traits_type::char_type must be the same type as CharT");
+
 	// [string.view.cons], construct/copy
 	_LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
 	basic_string_view() _NOEXCEPT : __data (nullptr), __size(0) {}