[I apologize for my mistakes, I wrote the email very quickly]
2 things here:
1. "constexpr" would ideally need to be treated like a qualifier.
This way it could propagate like cv qualifier.
constexpr is not part of the type system, it cannot be treated like qualifier. Given:
constexpr int i = 1;
int const j = 2;
The types of i and j are the same, both int const. There is a proposal (P1045) for constexpr parameters, but there constexpr would work very differently than the way qualifiers work: a constexpr parameter would [have to] behave like a template.
If you want to "propagate" constexpr, you just declare your function constexpr. constexpr is already conditionally constexpr.
Barry