[PATCH] libstdc++: add a workaround for format_kind> [PR120644]

[Giuseppe D'Angelo]

Hi,

Attached is a workaround for

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120644




Thank you,

--
Giuseppe D'Angelo

From 2e9da102644c9f74b120e9f4504562ab7a19df8f Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
Date: Fri, 13 Jun 2025 11:38:23 +0200
Subject: [PATCH] libstdc++: add a workaround for format_kind<optional<T>>
 [PR120644]

The specialization of format_kind for optional is causing a problem when
optional is imported and included. The comments on the PR strongly
suggest that this is a frontend bug; this commit just works around the
issue by specifying the type of format_kind<optional<T>> to be
`range_format`, rather than leaving the compiler deduce it via `auto`.

	PR c++/120644

libstdc++-v3/ChangeLog:

	* include/std/optional (format_kind): Do not use `auto`.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
---
 libstdc++-v3/include/std/optional | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 2ae71f11e21..cc7af5bbd7d 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1815,7 +1815,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       ranges::enable_view<optional<_Tp>> = true;
 
   template<typename _Tp>
-    inline constexpr auto
+    inline constexpr range_format
       format_kind<optional<_Tp>> = range_format::disabled;
 #endif // __cpp_lib_optional_range_support
 
-- 
2.43.0

It looks it's a C++ frontend bug, this commit just silences the regression in the modules testsuite that have been observed after the introduction of range support for optional in r16-1487-g774ae8734f0e19. The semantic of the code is otherwise equivalent -- the underlying bug is not addressed by this.

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