[Bug c++/115900] [14/15 Regression] constexpr object modification during construction gives "Modifying a const object is not allowed in a constant expression"

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115900

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Reverting r14-409 fixes the problem on trunk, so that change must mean
something.

I added the modifying-const-objects constexpr checking, so maybe I should poke
more at this.