[Bug libstdc++/121929] std::format prints a valid-looking date for invalid 2020y/January/Friday[10]

[tkaminsk at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121929

--- Comment #1 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
weekday_index constructed out of Friday[10] has unspecified value, but index
value in range [0, 7] are supported.