remaining std.format bugs

[Bruno Haible]

Hi,

2 out of the 4 bugs regarding format string processing in the 
std.format package are still open:
   - https://github.com/dlang/phobos/issues/10711
   - https://github.com/dlang/phobos/issues/10712

Their impact is not only that
   - some format strings written according to the D documentation 
https://dlang.org/library/std/format.html produce an exception at 
runtime,

but also that

   - the format string checking in 'msgfmt -c' (which is based on 
the D documentation) may consider some format strings as valid 
that will produce an exception at runtime.