Sorry. It is -fimplicit-constexpr, new in gcc12: https://gcc.gnu.org/gcc-12/changes.html

So that doesn't implicitly mark all functions as constexpr. It implicitly marks inline functions as constexpr. It weakens your point a tad, but I see now what you are getting at.

Yes sorry, I should have been more precise.

Of course you need the function body to actually do comptime evaluation.