[gcc r11-8621] libstdc++: Replace incorrect static assertion in std::reduce [PR95833]

[Jonathan Wakely]

https://gcc.gnu.org/g:fe918041ce9809755a7e1b2f62071c2b27829610

commit r11-8621-gfe918041ce9809755a7e1b2f62071c2b27829610
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 f36c1f96b2e..2ab337a0f3a 100644
--- a/libstdc++-v3/include/std/numeric
+++ b/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)
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{});