[Bug c++/68588] GCC requires constexpr for template non-type parameter, even though constexpr conversion operator exists

[petr at kalinin dot nnov.ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68588

--- Comment #1 from pkalinin <petr at kalinin dot nnov.ru> ---
[Attribution was incorrect, the last sentence is reworded from Columbo's
answer. Sorry.]