why is arch-x86_32/arch-x86_64 marked with flags: compiler? also msys2* and system-mingw/system-msvc

[hannesm]

//cc @dra27 @kit-ty-kate -- related to dra27#21

[dra27]

opam 2.0 complains if you specify base packages which aren’t flagged compiler. It became less relevant in opam 2.1 and IIRC in opam 2.2 we got rid of all remaining interpretations of the compiler flag in opam itself

[dra27]

Oops - a pre-caffeinated answer. The removal I was referring to is mentions of base packages, not the interpretation of the compiler flag itself. The answer for why thoe msys2, arch- and system- packages are flagged compiler is opam 2.0 base package-related. For the issue in dra27#21, the issue is that opam uses the compiler to infer the switch invariant.

[hannesm]

Thanks for your reply @dra27 - I still not fully understand that, but if it is needed for opam 2.0, then let's leave it as is.

[hannesm]

FWIW, this pollutes the opam switch list-available quite a bit:

# Listing available compilers from repositories: default
# Name                                 # Version                              # Synopsis
msys2-clang32                          1                                      MSYS2 CLANG32 Environment
msys2-clang64                          1                                      MSYS2 CLANG64 Environment
msys2-clangarm64                       1                                      MSYS2 CLANGARM64 Environment
msys2-mingw32                          1                                      MSYS2 MINGW32 Environment
msys2-mingw64                          1                                      MSYS2 MINGW64 Environment
msys2-ucrt64                           1                                      MSYS2 UCRT64 Environment
ocaml-option-afl                       1                                      Set OCaml to be compiled with afl-fuzz instrumentation
...

[kit-ty-kate]

Since those packages are windows related and opam 2.0 doesn't support windows it feels unnecessary to have that flag for those packages

[dra27]

Cross-referencing #17541 which confirms I had remembered correctly! I agree with @kit-ty-kate, though, for the Windows-specific msys2- ones. I'll try to get a PR done for it (the slow part is getting round to testing it...!)