fix #47658, state stack overflow on unions containing typevars

[JeffBezanson]

I intended this to be a starting point for experimenting, and was astonished that it actually worked well. It may be subtly wrong; in particular I think that when this heuristic check returns true we need to come back and try the less efficient algorithm if subtyping later fails to hold. I'll try to construct a counterexample. In the meantime it's nice to see so many tests newly passing.

@nanosoldier runtests(ALL, vs=":master")

[JeffBezanson]

@nanosoldier runtests(ALL, vs=":master")

[maleadt]

It's better to use the simplified PkgEval invocation:

@nanosoldier runtests()

That will automatically default to ALL packages, as well as use the merge-base against master for a fairer comparison.

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected.
A full report can be found here.

[JeffBezanson]

Wow, pretty good results:

InformationGeometry and QuantumTomography: looks like numerical issues / random tests
Unitful: unexpected pass, fixes actual bug
Zarr: network issue?

[maleadt]

Zarr: network issue?

I'll add that one (connect: connection refused) to the network issue regex.

[JeffBezanson]

Ok yeah it's not that hard to fool the greedy algorithm:

julia> Tuple{Ref{Union{Int,String}}, Ref{Union{Int,String}}} <: Tuple{Ref{Union{X,Y}}, Ref{X}} where {X,Y}
false

Unfortunately this slightly more complicated case is also wrong on master:

julia> Tuple{Ref{Union{Int,String,Missing}}, Ref{Union{Int,String}}} <: Tuple{Ref{Union{X,Y}}, Ref{X}} where {X,Y}
false

Seems to have been that way at least since 1.0. (Edit: simply had to check, and amazingly this works on 0.6.4!!)

[JeffBezanson]

I think this is ready to go.

[vtjnash]

Sounds good to me. But why isn't forall_exists_equal being strictly more expensive than <:, when it supposedly calls both <: and >:

[JeffBezanson]

It is a bit perverse, but I think what's happening is that yes, doing both <: and >: is more expensive, but that makes it worthwhile to put in a fast path, so in this case it gets faster. We could probably add more fast paths like this elsewhere too.