https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86524
David Stone <david at doublewise dot net> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |david at doublewise dot net --- Comment #1 from David Stone <david at doublewise dot net> --- I am not sure that the problem is actually in libstdc++. I would expect that any implementation of std::less that is valid in f1 would also be valid in f2. Declaring the function itself constexpr should have no bearing on the static_assert.