Proposed resolution:

This wording is relative to N4431.

1. Change [alg.sorting]/3 to the following:

   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 25.4.3 to work correctly, comp shallhas to induce a strict weak ordering on the values.

[ 2015-05, Lenexa ]

Move to Immediate.

N4296 [alg.sorting]/3 reads:

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 25.4.3 to work correctly, comp has to induce a strict weak ordering on the values.

So it's not specified clearly what happens if comp or operator< don't induce a strict weak ordering. Is it undefined or implementation-defined behavior? It seems that it should be stated more clearly that the behavior is undefined.