Enable -Wconversion for GHC's C code
Because apparently none of
-pedantic of GCC imply
-Wconversion or its subset
That means currently you can call a function taking an
unsigned int with a
signed int without getting any warning.
That is very bad, because of course negative numbers will overflow.
-Wconversion would have caught overflow errors such as this one: https://phabricator.haskell.org/D3955#110993
We should probably have it for the C code.
Turning it on everywhere will be quite some work though because it creates lots of complaints such as
long long int not being the same as
long int (which is technically correct but in practice they are typically the same).