Implement string_view from the library fundamentals TS (n4023). Also works in C++11 and 03, with reduced functionality (mostly in the area of constexpr)

llvm-svn: 210659
diff --git a/libcxx/test/experimental/string.view/string.view.access/front.pass.cpp b/libcxx/test/experimental/string.view/string.view.access/front.pass.cpp
new file mode 100644
index 0000000..f53dfb77
--- /dev/null
+++ b/libcxx/test/experimental/string.view/string.view.access/front.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 const _CharT& back();
+
+#include <experimental/string_view>
+#include <cassert>
+
+template <typename CharT>
+bool test ( const CharT *s, size_t len ) {
+    std::experimental::basic_string_view<CharT> sv ( s, len );
+    assert ( sv.length() == len );
+    assert ( sv.front() == s[0] );
+    return &sv.front() == s;
+    }
+    
+int main () {
+    assert ( test ( "ABCDE", 5 ));
+    assert ( test ( "a", 1 ));
+
+    assert ( test ( L"ABCDE", 5 ));
+    assert ( test ( L"a", 1 ));
+
+#if __cplusplus >= 201103L
+    assert ( test ( u"ABCDE", 5 ));
+    assert ( test ( u"a", 1 ));
+
+    assert ( test ( U"ABCDE", 5 ));
+    assert ( test ( U"a", 1 ));
+#endif
+
+#if _LIBCPP_STD_VER > 11
+    {
+    constexpr std::experimental::basic_string_view<char> sv ( "ABC", 2 );
+    static_assert ( sv.length() ==  2,  "" );
+    static_assert ( sv.front()  == 'A', "" );
+    }
+#endif
+}