Testing Diagonalization asserts on 1.8 in subtyping

[KristofferC]

https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/8b2e406_vs_742b9ab/Diagonalizations.primary.log

(jl_2ZlRez) pkg> test Diagonalizations
....

signal (6): Aborted
in expression starting at /home/kc/.julia/packages/Diagonalizations/VKvpN/test/runtests.jl:93
pthread_kill at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
raise at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
abort at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 0x7fdc5af2171a)
__assert_fail at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
ijl_subtype_env at /home/kc/julia1.8/src/subtype.c:1878
subtype_tuple_tail at /home/kc/julia1.8/src/subtype.c:1076 [inlined]
subtype_tuple at /home/kc/julia1.8/src/subtype.c:1158 [inlined]
subtype at /home/kc/julia1.8/src/subtype.c:1298
exists_subtype at /home/kc/julia1.8/src/subtype.c:1394
forall_exists_subtype at /home/kc/julia1.8/src/subtype.c:1422
ijl_types_equal at /home/kc/julia1.8/src/subtype.c:1969
jl_specializations_get_linfo_ at /home/kc/julia1.8/src/gf.c:129
#specialize_method#255 at ./compiler/utilities.jl:204
specialize_method at ./compiler/utilities.jl:190 [inlined]
typeinf_edge at ./compiler/typeinfer.jl:823
abstract_call_method at ./compiler/abstractinterpretation.jl:643
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:154
abstract_call_known at ./compiler/abstractinterpretation.jl:1674
...

Also happens for PosDefManifoldML.jl

[JeffBezanson]

Reduced:

T = TypeVar(:T,Real)
V = TypeVar(:V,AbstractVector{T})
x = UnionAll(T, UnionAll(V, UnionAll(T, Type{Pair{T, V}})))
y = UnionAll(T, UnionAll(V, Type{Pair{T, V}}))
x <: y

So we have a nested UnionAll using the same TypeVar, which confuses obvious_subtype.

[vtjnash]

That confuses me too haha. I suspect this would confused obviously_egal too, since we normally assume == (or egal) implies <:, but here that is false. Neither should really need that case, though it may get worse, as we also could have this, and it is much more unfortunate that we must recurse into check the contents of S here:

T = TypeVar(:T,Real)
V = TypeVar(:V,AbstractVector{T})
S = Type{Pair{T, V}}
x = UnionAll(T, UnionAll(V, UnionAll(T, S)))
y = UnionAll(T, UnionAll(V, S))
x <: y