Add forward declaration of operator<< in <string_view> as required.

This declaration was previously missing despite appearing in the
synopsis. Users are still required to include <ostream> to get the
definition of the streaming operator.

llvm-svn: 372909

1 files changed, 26 insertions, 0 deletions

diff --git a/libcxx/test/std/strings/string.view/string.view.io/stream_insert_decl_present.pass.cpp b/libcxx/test/std/strings/string.view/string.view.io/stream_insert_decl_present.pass.cpp
new file mode 100644
index 00000000000..be0567a7372
--- /dev/null
+++ b/libcxx/test/std/strings/string.view/string.view.io/stream_insert_decl_present.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// template<class charT, class traits, class Allocator>
+//   basic_ostream<charT, traits>&
+//   operator<<(basic_ostream<charT, traits>& os,
+//              const basic_string_view<charT,traits> str);
+
+#include <string_view>
+#include <iosfwd>
+
+template <class SV, class = void>
+struct HasDecl : std::false_type {};
+template <class SV>
+struct HasDecl<SV, decltype(static_cast<void>(std::declval<std::ostream&>() << std::declval<SV&>()))> : std::true_type {};
+
+int main() {
+  static_assert(HasDecl<std::string_view>::value, "streaming operator declaration not present");
+}