libstdc++: Avoid hard error in ranges::unique_copy [PR100770]

Here, in the constexpr if condition within ranges::unique_copy, when
input_iterator<_Out> isn't satisfied we must avoid substituting into
iter_value_t<_Out> because the latter isn't necessarily well-formed
then.  To that end, this patch factors out the condition into a concept
and uses it throughout.

This patch also makes the definition of our testsuite
output_iterator_wrapper more minimal by setting its value_type, pointer
and reference member types to void.  This means our existing tests for
unique_copy already exercise the fix for this bug, so we don't need
to add another test.  The only other fallout of this testsuite iterator
change appears in std/ranges/range.cc, where the use of range_value_t
on a test_output_range is now ill-formed.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (__detail::__can_reread_output):
	Factor out this concept from ...
	(__unique_copy_fn::operator()): ... here.  Use the concept
	throughout.
	* testsuite/std/ranges/range.cc: Remove now ill-formed use
	of range_value_t on an output_range.
	* testsuite/util/testsuite_iterators.h (output_iterator_wrapper):
	Define value_type, pointer and reference member types to void.

Author: Patrick Palka

libstdc++-v3/include/bits/ranges_algo.h

@@ -1396,6 +1396,13 @@ namespace ranges
 
   inline constexpr __unique_fn unique{};
 
+  namespace __detail
+  {
+    template<typename _Out, typename _Tp>
+      concept __can_reread_output = input_iterator<_Out>
+	&& same_as<_Tp, iter_value_t<_Out>>;
+  }
+
   template<typename _Iter, typename _Out>
     using unique_copy_result = in_out_result<_Iter, _Out>;
 
@@ -1407,8 +1414,7 @@ namespace ranges
 	       projected<_Iter, _Proj>> _Comp = ranges::equal_to>
       requires indirectly_copyable<_Iter, _Out>
 	&& (forward_iterator<_Iter>
-	    || (input_iterator<_Out>
-		&& same_as<iter_value_t<_Iter>, iter_value_t<_Out>>)
+	    || __detail::__can_reread_output<_Out, iter_value_t<_Iter>>
 	    || indirectly_copyable_storable<_Iter, _Out>)
       constexpr unique_copy_result<_Iter, _Out>
       operator()(_Iter __first, _Sent __last, _Out __result,
@@ -1432,8 +1438,7 @@ namespace ranges
 		}
 	    return {__next, std::move(++__result)};
 	  }
-	else if constexpr (input_iterator<_Out>
-			   && same_as<iter_value_t<_Iter>, iter_value_t<_Out>>)
+	else if constexpr (__detail::__can_reread_output<_Out, iter_value_t<_Iter>>)
 	  {
 	    *__result = *__first;
 	    while (++__first != __last)
@@ -1467,8 +1472,7 @@ namespace ranges
 	       projected<iterator_t<_Range>, _Proj>> _Comp = ranges::equal_to>
       requires indirectly_copyable<iterator_t<_Range>, _Out>
 	&& (forward_iterator<iterator_t<_Range>>
-	    || (input_iterator<_Out>
-		&& same_as<range_value_t<_Range>, iter_value_t<_Out>>)
+	    || __detail::__can_reread_output<_Out, range_value_t<_Range>>
 	    || indirectly_copyable_storable<iterator_t<_Range>, _Out>)
       constexpr unique_copy_result<borrowed_iterator_t<_Range>, _Out>
       operator()(_Range&& __r, _Out __result,

libstdc++-v3/testsuite/std/ranges/range.cc

@@ -75,9 +75,6 @@ static_assert( same_as<std::ranges::range_difference_t<C>,
 static_assert( same_as<std::ranges::range_difference_t<O>,
 		       std::ptrdiff_t> );
 
-static_assert( same_as<std::ranges::range_value_t<O>,
-		       char> );
-
 static_assert( same_as<std::ranges::range_reference_t<I>,
 		       char&> );
 static_assert( same_as<std::ranges::range_reference_t<O>,

libstdc++-v3/testsuite/util/testsuite_iterators.h

@@ -122,7 +122,7 @@ namespace __gnu_test
    */
   template<class T>
   struct output_iterator_wrapper
-  : public std::iterator<std::output_iterator_tag, T, std::ptrdiff_t, T*, T&>
+  : public std::iterator<std::output_iterator_tag, void, std::ptrdiff_t, void, void>
   {
   protected:
     output_iterator_wrapper() : ptr(0), SharedInfo(0)