conversion and promotion to floats

[putianyi889]

No description provided.

[coveralls]

coverage: 91.463% (-1.5%) from 93.004%
when pulling c0ba93e on putianyi889:patch-1
into 26f27c3 on anj1:master.

[fkastner]

Hi,
thanks for your interest in this project.
I'm not convinced that we want to have this promote_rule. Could you provide your rationale for choosing T over AlgebraicNumber as promoted type?
Also I would probably sort that under the existing "promotions" heading.

Less importantly, I wonder if we should provide a nicer error message for the case where one tries to cast a genuinely complex-valued AlgebraicNumber to float. But maybe the InexactError is already descriptive enough.

[putianyi889]

Could you provide your rationale for choosing T over AlgebraicNumber as promoted type?

I think it's the same as irrationals.
https://github.com/JuliaLang/julia/blob/64de065a183ac70bb049f7f9e30d790f8845dd2b/base/irrationals.jl#L44-L52

Also I would probably sort that under the existing "promotions" heading.

The top comments of the file says it's meant to only support integers, but I could edit that anyway.

to cast a genuinely complex-valued AlgebraicNumber to float

I forgot algebraic numbers are complex. I'll add complex promotions as well.