[gcc r9-9594] libstdc++: Replace incorrect static assertion in std::reduce [PR95833]

[Jonathan Wakely]

https://gcc.gnu.org/g:ae17634d5fce496a50477eb0199f1eda8249622f

commit r9-9594-gae17634d5fce496a50477eb0199f1eda8249622f
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 7f80d06cfbc..25fdfaf735f 100644
--- a/libstdc++-v3/include/std/numeric
+++ b/libstdc++-v3/include/std/numeric
@@ -261,9 +261,11 @@ _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{});