Promotion of Irrational{s} with Complex{Irrational{s}} gives StackOverflowError

[jmbyrne]

The promote_rule with an Irrational{s} type and a Complex{Irrational{s}} type (where s is the same Symbol) gives a StackOverflowError. The intended type is probably Complex{Irrational{s}}, as demonstrated by swapping the arguments:

If the parameters are different (e.g. Irrational{:ℯ} with Complex{Irrational{:π}}), then there is no issue as it promotes to ComplexF64.

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Julia version 1.9.2 installed on Windows 11 with the Julia installer https://julialang.org/downloads/

[Seelengrab]

A bit orthogonal, but please note that Julia is not a CAS - not everything is expected to preserve irrationals, or combinations thereof. This looks like it should be fixed though, as simply swapping the order of arguments already works.

[jmbyrne]

I agree that since Irrational types are singleton types it would be very rare to actually come across this. However, it is possible, for instance promote(π, complex(π, π)) causes the same error. Not sure why you'd use complex(π, π), but there might be a good reason!

For a bit of added context, I was trying to promote every possible pair of numeric types to generate a diagram, but this was stopping it. Since you mentioned swapping the arguments, there is some more strange behaviour related to that, e.g.:

I don't know whether this is actually a problem, and if it is, whether it should be its own issue anyway. I'll let someone else be the judge of that.

[bvdmitri]

I think you are supposed to use promote_type instead, which uses the promote_rule under the hood but also ensures symmetry.

[jmbyrne]

You're right, I missed that. The original issue still stands though, that breaks with either promote_type or promote_rule.

[nsajko]

Faulty rule:

julia/base/irrationals.jl

Line 48 in 25cbe00

promote_rule(::Type{S}, ::Type{T}) where {S<:AbstractIrrational,T<:Number} = promote_type(promote_type(S, real(T)), T)

xref #55866

[mbauman]

Is this the same as #13193?

[nsajko]

No, this one should be easier to fix.