Promotion system for Galois fields not properly implemented?

[HechtiDerLachs]

When I was trying to implement the ring interface for Oscar, I stumbled upon the following oddity:

promote_rule(fmpz, Int)

gives fmpz as expected. But

promote_rule(gfp_elem, Int)

returns Union{}. That's not correct, is it?

This seems to be breaking my tests...

[tthsqe12]

The gfp_elem type is defined in Nemo, so it should end up there. But if it is stalling your oscar tests, maybe put it in oscar with a TODO: move this to nemo

[HechtiDerLachs]

I'm trying to find the correct promote_rules to add, but I don't get through. So, it would be nice if someone could have a look at this with me or just provide the fix.

Tests just don't cover the Galois fields for now. But they should in the future.

[HechtiDerLachs]

Alright, now it seems I could fix it. Turns out that promotion for gfp_mpoly was also not there, yet. The lines I need to add to make my tests run are

AbstractAlgebra.promote_rule(::Type{gfp_elem}, ::Type{fmpz}) = gfp_elem
AbstractAlgebra.promote_rule(::Type{gfp_mpoly}, ::Type{fmpz}) = gfp_mpoly

I could add them to my files with the TODO. But in any case: Could someone double check that these are the correct lines? For instance, I'm afraid, the promotion of Int64 is not covered by these, is it?

[thofma]

This looks good. I will add them to AA today. The Int/Integer one would be

AbstractAlgebra.promote_rule(::Type{gfp_mpoly}, ::Type{<: Integer}) = gfp_mpoly

[thofma]

But you can add them here too and we sort things out later. This should not hold up your PR.

[fingolfin]

A variation of this still seems to be in play?

julia> Oscar.promote_rule(FqFieldElem, Int)
Union{}

julia> promote_rule(FqFieldElem, ZZRingElem)
Union{}

But I am not sure what exactly this breaks... ?

[fingolfin]

Also

julia> promote_rule(fpFieldElem, Int)
Union{}

etc.

@thofma says "all of these should be added". So we just have to find/assign someone to do it.

[fingolfin]

@thofma just naively volunteered (har har)

[thofma]

Oscar.promote_rule is not what you want. It is working correctly with the correct function

julia> Nemo.promote_rule(FqFieldElem, Integer)
FqFieldElem

julia> Nemo.promote_rule(FqFieldElem, ZZRingElem)
FqFieldElem