Rationale (April, 2018):

The design was thoroughly discussed before adopting the feature and the current specification was intentionally adopted. Any request for a change should go through the normal EWG process at this point.

The effect of constexpr if in non-templated code is primarily limited to not requiring definitions for entities that are odr-used in discarded statements. This eliminates a plausible implementation techique of simply skipping the tokens of a discarded statement. Should the Standard allow such an approach? One needed change might be to say that all diagnosable rules become “no diagnostic required” inside discarded statements.