GCC 6.4.0 compilation errors

[sergiud]

Compiling fmt 11.1.2 using GCC 6.4.0 triggers compilation errors caused by the bigint class. Reproducer is here. There are also additional errors related to uninitialized buffer of char in constexpr context that do not appear the repro but are triggered during the compilation of unit tests.

[vitaut]

Should be fixed in 41539c2, thanks for reporting!