[tsimionescu]

When I say "a constexpr function", I mean "a function that is legally constexpr-qualified". An expression calling such a function with a constant argument is a core constant expression, as long as the evaluation of the function given that argument doesn't produce UB or leak memory.

But, this tells us nothing about whether calling the same function with some other argument would produce UB or not. Unless we try to form a core constant expression containing a call to that function for every possible argument value, we can't be sure if the function is free of UB for every possible input value.

My point, more simply, is: the fact that foo(1) is verified by the compiler to be a core constant expression is not a guarantee that foo(2) would be a core constant expression as well. Your original comment seems to me to imply that it does.

The consequence of this is that, as said elsewhere, this trick only helps to ensure that a unit test is not accidentally passing because of UB (and that memory is not leaked for that input value). It doesn't help provide any other guarantees.