libstdc++: Replace incorrect static assertion in std::reduce [PR95833]

The standard does not require the iterator's value type to be
convertible to the result type, it only requires that the result of
dereferencing the iterator can be passed to the binary function.

libstdc++-v3/ChangeLog:

	PR libstdc++/95833
	* include/std/numeric (reduce(Iter, Iter, T, BinaryOp)): Replace
	incorrect static_assert with ones matching the 'Mandates'
	conditions in the standard.
	* testsuite/26_numerics/reduce/95833.cc: New test.

libstdc++-v3/include/std/numeric

@@ -278,9 +278,11 @@ namespace __detail
     reduce(_InputIterator __first, _InputIterator __last, _Tp __init,
 	   _BinaryOperation __binary_op)
     {
-      using value_type = typename iterator_traits<_InputIterator>::value_type;
+      using __ref = typename iterator_traits<_InputIterator>::reference;
+      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, _Tp&, __ref>);
+      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref, _Tp&>);
       static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, _Tp&, _Tp&>);
-      static_assert(is_convertible_v<value_type, _Tp>);
+      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref, __ref>);
       if constexpr (__is_random_access_iter<_InputIterator>::value)
 	{
 	  while ((__last - __first) >= 4)

libstdc++-v3/testsuite/26_numerics/reduce/95833.cc

@@ -0,0 +1,16 @@
+// { dg-do compile { target c++17 } }
+// PR libstdc++/95833 - Incorrect static_assert in std::reduce overload
+
+#include <numeric>
+
+struct A { };
+struct B { };
+
+struct binop
+{
+  template<typename T, typename U>
+    A operator()(T&&, U&&) const { return A{}; }
+};
+
+B b;
+A a = std::reduce(&b, &b + 1, A{}, binop{});