Compiling and running core tests with libc++ with debug hardening mode produces following error (Core_reduceArgMinMax):

/usr/local/include/c++/v1/__algorithm/comp_ref_type.h:47: 
    assertion !__comp_(__l, __r) failed: Comparator does not induce a strict weak ordering

Perhaps we should not use std::min_element with std::less_equal. E.g. C++-17 (?) (N4659) standard says:

28.7

3. For all algorithms that take Compare, there is a version that uses operator< instead.
   That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms other than those
   described in 28.7.3 (binary_search etc.), comp shall induce a strict weak ordering on the values.

This might look a bit pedantic, but since this is just test I think we can fix this problem.