[gcc r12-7003] libstdc++: Fix link failure in _OutputIteratorConcept

[Jonathan Wakely]

https://gcc.gnu.org/g:b229c5186093fa6603030ae83d5fe640ef7051a4

commit r12-7003-gb229c5186093fa6603030ae83d5fe640ef7051a4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jan 25 21:29:31 2022 +0000

    libstdc++: Fix link failure in _OutputIteratorConcept
    
    The C++98-style concept check for output iterators causes a link
    failure on mingw-w64, because the __val() member function isn't defined.
    Change it to use a function pointer instead. That pointer is never set
    to anything meaningful, but it doesn't matter as the __constraints()
    function only has to be instantiated, it's never called.
    
    We could refactor all of these to use unevaluated contexts (e.g. sizeof
    of __decltype) so that we only check the expressions are well-formed,
    without any codegen at all. Any improvements to these are very low
    priority though.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/boost_concept_check.h (_OutputIteratorConcept):
            Change member function to data member of function pointer type.

Diff:
---
 libstdc++-v3/include/bits/boost_concept_check.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/boost_concept_check.h b/libstdc++-v3/include/bits/boost_concept_check.h
index 23dba067693..3d884eb440f 100644
--- a/libstdc++-v3/include/bits/boost_concept_check.h
+++ b/libstdc++-v3/include/bits/boost_concept_check.h
@@ -483,7 +483,9 @@ struct _Aux_require_same<_Tp,_Tp> { typedef _Tp _Type; };
       *__i++ = __val();                 // require postincrement and assignment
     }
     _Tp __i;
-    _ValueT __val() const;
+    // Use a function pointer here so no definition of the function needed.
+    // Just need something that returns a _ValueT (which might be a reference).
+    _ValueT (*__val)();
   };
 
   template<typename _Tp>