[gcc r13-4614] libstdc++: Fix constraint on std::basic_format_string [PR108024]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:6c0f9584013dbedeacacb041d40ce9005b902df0

commit r13-4614-g6c0f9584013dbedeacacb041d40ce9005b902df0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 12 11:40:07 2022 +0000

    libstdc++: Fix constraint on std::basic_format_string [PR108024]
    
    Also remove some redundant std::move calls for return statements.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/108024
            * include/std/format (basic_format_string): Fix constraint.
            * testsuite/std/format/format_string.cc: New test.

Diff:
---
 libstdc++-v3/include/std/format                    | 10 ++++++----
 libstdc++-v3/testsuite/std/format/format_string.cc | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 269f8f6cc6a..9c928371415 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -110,7 +110,8 @@ namespace __format
   template<typename _CharT, typename... _Args>
     struct basic_format_string
     {
-      template<convertible_to<basic_string_view<_CharT>> _Tp>
+      template<typename _Tp>
+	requires convertible_to<const _Tp&, basic_string_view<_CharT>>
 	consteval
 	basic_format_string(const _Tp& __s);
 
@@ -609,7 +610,7 @@ namespace __format
       else
 	for (_CharT __c : __str)
 	  *__out++ = __c;
-      return std::move(__out);
+      return __out;
     }
 
   // Write STR to OUT with NFILL copies of FILL_CHAR specified by ALIGN.
@@ -664,7 +665,7 @@ namespace __format
       __out = __format::__write(std::move(__out), __str);
       __pad(__r, __out);
 
-      return std::move(__out);
+      return __out;
     }
 
   // A lightweight optional<locale>.
@@ -3626,7 +3627,8 @@ namespace __format
 /// @endcond
 
   template<typename _CharT, typename... _Args>
-    template<convertible_to<basic_string_view<_CharT>> _Tp>
+    template<typename _Tp>
+      requires convertible_to<const _Tp&, basic_string_view<_CharT>>
       consteval
       basic_format_string<_CharT, _Args...>::
       basic_format_string(const _Tp& __s)
diff --git a/libstdc++-v3/testsuite/std/format/format_string.cc b/libstdc++-v3/testsuite/std/format/format_string.cc
new file mode 100644
index 00000000000..1dd6ca8f125
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/format/format_string.cc
@@ -0,0 +1,16 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <format>
+
+struct Str
+{
+  consteval operator std::string_view() const { return ""; }
+  operator std::string_view() = delete;
+};
+
+// PR libstdc++/108024
+static_assert( std::is_constructible_v<std::format_string<>, const Str&> );
+static_assert( std::is_convertible_v<const Str&, std::format_string<>> );
+
+constinit std::format_string<> s = Str();