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 --git a/libstdc++-v3/include/bits/boost_concept_check.h b/libstdc++-v3/include/bits/boost_concept_check.h
index 23dba06..3d884eb 100644 (file)
--- 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>