US171 17.11.02.1 p4 Define "substitutability" properly

[wg21bot]

Substitutability is ill-defined because it circularly depends on “comparison-salient state” and it is itself used to determine the correct return time of comparisons. Comparisons define what is “comparison-salient”, and if f can distinguish between a and b, it must be examining state that the comparison did not consider salient.

Proposed change:
Either clarify the definition of substitutability, or eliminate the distinctions between strong and weak comparisons, which are the only place that definition is used.

[dietmarkuehl]

Reviewed Monday afternoon in Belfast in LWG small group session.

[mclow]

Reviewed Monday in Prague. No consensus for change for C++20. Will create LWG issue to track this post-C++20

[jensmaurer]

Rejected. There was no consensus to adopt this change.