[gcc r15-7896] libstdc++: Add poison pill for chrono::from_stream

[Jonathan Wakely]

https://gcc.gnu.org/g:4b180459626df64dac758da8760d868c2604ddf2

commit r15-7896-g4b180459626df64dac758da8760d868c2604ddf2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 6 20:23:29 2025 +0000

    libstdc++: Add poison pill for chrono::from_stream
    
    LWG 3956 (approved in Hagenberg, February 2025) decided that from_stream
    should be found *only* by ADL, not ordinary unqualified lookup. Add a
    poison pill overload to chrono::__detail where the __parsable concept
    and operator>>(basic_istream&, _Parser) are defined. This ensures that
    when they use from_stream unqualified ordinary lookup finds the poison
    pill, which is deleted, so a usable overload resolution result can only
    be found by ADL.
    
    We already have the std/time/parse/parse.cc test checking that ADL
    works, so this doesn't add a new test.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/chrono_io.h (chrono::__detail::from_stream): Add
            deleted function as poison pill for unqualified lookup.
    
    Reviewed-by: Patrick Palka <ppalka@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 8c026586d4c9..c16b555df290 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -3115,6 +3115,10 @@ namespace __detail
 
 namespace __detail
 {
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3956. chrono::parse uses from_stream as a customization point
+  void from_stream() = delete;
+
   template<typename _Parsable, typename _CharT,
 	   typename _Traits = std::char_traits<_CharT>,
 	   typename... _OptArgs>