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.
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