[gcc r10-9945] libstdc++: Replace incorrect static assertion in std::reduce [PR95833]

[Jonathan Wakely]

https://gcc.gnu.org/g:3a37d5c0bd3451104ddab85f69ec37b50d31fa7f

commit r10-9945-g3a37d5c0bd3451104ddab85f69ec37b50d31fa7f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jun 18 14:46:58 2021 +0100

    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:
---
 libstdc++-v3/include/std/numeric                   |  6 ++++--
 libstdc++-v3/testsuite/26_numerics/reduce/95833.cc | 16 ++++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/numeric b/libstdc++-v3/include/std/numeric
index 6ad73d9713b..03cf719c676 100644
--- 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
index 00000000000..cf4644f53c9
--- /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{});