@skrah, could you take look on this variant of solving #108562? Attempt to enable -fstrict=overflow was unsuccessful, unfortunately.

@vstinner, maybe you could review this little pr? Note, that 4.0.0 has more diagnostic ignored (at top of the file):

#if defined(__GNUC__) && !defined(__INTEL_COMPILER) && __GNUC__ >= 7
  #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
  #pragma GCC diagnostic ignored "-Wmisleading-indentation"
  #pragma GCC diagnostic ignored "-Warray-bounds"
#endif

I turn of one diagnostic and more locally (with following pop directive).

Another option (mentioned in the issue) - update bundled copy.

Modules/_decimal/libmpdec/io.c

libmpdec/ is vendored copy of a 3rd party project. You should fix the issue in the upstream first. Otherwise, it will be lost at the next update.

See upstream: https://www.bytereef.org/mpdecimal/.

Ok, then I close pr and issue.

Unfortunately, @skrah blocked in python's repos. But he is aware about issue anyway.

Merged, thank you.