[committed] libstdc++: Add default template argument to basic_istream_view

[Jonathan Wakely via Gcc-patches]

The standard shows this default template argument in the <ranges>
synopsis, but it was missing in libstdc++.

libstdc++-v3/ChangeLog:

        * include/std/ranges (basic_istream_view): Add default template
        argument.
        * testsuite/std/ranges/istream_view.cc: Check it.

Tested x86_64-linux. Committed to trunk. Backports to follow.


commit 1a129376bbc26d3c30af3c1ae6036e2e1446db40
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Aug 23 16:16:05 2021

    libstdc++: Add default template argument to basic_istream_view
    
    The standard shows this default template argument in the <ranges>
    synopsis, but it was missing in libstdc++.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (basic_istream_view): Add default template
            argument.
            * testsuite/std/ranges/istream_view.cc: Check it.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 3d49a26ee79..b373e4f05c0 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -679,7 +679,8 @@ namespace views
        = requires(basic_istream<_CharT, _Traits>& is, _Val& t) { is >> t; };
   } // namespace __detail
 
-  template<movable _Val, typename _CharT, typename _Traits>
+  template<movable _Val, typename _CharT,
+          typename _Traits = char_traits<_CharT>>
     requires default_initializable<_Val>
       && __detail::__stream_extractable<_Val, _CharT, _Traits>
     class basic_istream_view
diff --git a/libstdc++-v3/testsuite/std/ranges/istream_view.cc 
b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
index 2f15f787250..f5c0c2a6bb0 100644
--- a/libstdc++-v3/testsuite/std/ranges/istream_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
@@ -94,6 +94,15 @@ test05()
     ;
 }
 
+void
+test06()
+{
+  // Default template argument
+  using V = std::ranges::basic_istream_view<int, char>;
+  using W = std::ranges::basic_istream_view<int, char, std::char_traits<char>>;
+  static_assert( std::is_same_v<V, W> );
+}
+
 int
 main()
 {
@@ -102,4 +111,5 @@ main()
   test03();
   test04();
   test05();
+  test06();
 }