I’ll admit I haven’t studied the words in the draft 202X standard closely for this, but it was my understanding that simple procedures were intended to be allowed in declarations in the way that only “constant expressions” were previously allowed, and that the constraints on simple procedures were sufficient to make them equivalent to constexpr. The standard may not require that a compiler take advantage of the ability to compute such functions at compile time in all cases, but that wouldn’t mean you couldn’t force it’s hand by declaring parameters to store the results.
Does anybody know of some reason that simple isn’t sufficient for use as an equivalent to constexpr? If so I’d be in favor of helping to correct any such deficiency before 202X is finished.