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.

(cherry picked from commit 0532452dcd17910dfd3d2b0df50dfe3ef1194bf7)

diff --git a/libstdc++-v3/include/std/numeric b/libstdc++-v3/include/std/numeric
index 6ad73d9713bbab25a56b13a3fd13fb7f0ad7c6bf..03cf719c676ea06512a8b34911bce6980d101658 100644 (file)
--- a/libstdc++-v3/include/std/numeric
+++ b/libstdc++-v3/include/std/numeric
@@ -259,9 +259,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     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)

diff --git a/libstdc++-v3/testsuite/26_numerics/reduce/95833.cc b/libstdc++-v3/testsuite/26_numerics/reduce/95833.cc
new file mode 100644 (file)
index 0000000..cf4644f
--- /dev/null
+++ b/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{});