Hard Crash: "Unreachable reached" when using the Subtyping Operator to compare against `Union{<: T}`

[Clemapfel]

@fjebaker discovered that this line hard-crashes Julia, tested on 1.7 and 1.8 on both Mac and Linux, though on Mac it does not signal(4) but hang indefinitely instead.

test(x::T) where T = T <: Union{<: Nothing}; test(12)

Unreachable reached at 0x7fbce15a9235

signal (4): Illegal instruction
in expression starting at REPL[2]:1
test at ./REPL[2]:1
unknown function (ip: 0x7fbce15a9259)
unknown function (ip: 0x7fbd46c6768b)
unknown function (ip: 0x7fbd46c66f56)
unknown function (ip: 0x7fbd46c67d92)
unknown function (ip: 0x7fbd46c68bdc)
unknown function (ip: 0x7fbd46c86919)
unknown function (ip: 0x7fbd46c87416)
unknown function (ip: 0x7fbd46c87416)
ijl_toplevel_eval_in at /usr/bin/../lib64/julia/libjulia-internal.so.1 (unknown line)
unknown function (ip: 0x7fbd250f9417)
unknown function (ip: 0x7fbd250f9b72)
unknown function (ip: 0x7fbd2488897d)
unknown function (ip: 0x7fbd2492d352)
unknown function (ip: 0x7fbd2492d773)
unknown function (ip: 0x7fbd2492d78f)
unknown function (ip: 0x7fbd24a8f20e)
unknown function (ip: 0x7fbd24a8f2ab)
jl_f__call_latest at /usr/bin/../lib64/julia/libjulia-internal.so.1 (unknown line)
unknown function (ip: 0x7fbd2550bb5e)
unknown function (ip: 0x7fbd25518cb5)
unknown function (ip: 0x7fbd250038d7)
unknown function (ip: 0x7fbd25003a08)
unknown function (ip: 0x7fbd46cb0926)
jl_repl_entrypoint at /usr/bin/../lib64/julia/libjulia-internal.so.1 (unknown line)
main at julia (unknown line)
__libc_start_call_main at /usr/bin/../lib64/libc.so.6 (unknown line)
__libc_start_main at /usr/bin/../lib64/libc.so.6 (unknown line)
_start at julia (unknown line)
Allocations: 2903 (Pool: 2891; Big: 12); GC: 0
Illegal instruction (core dumped)

It is unrelated to Nothing and the type of x, any other type in either place causes the crash.

More puzzling, the following works:

julia> x = 12; typeof(x) <: Union{<: Nothing}  # works
false

julia> test(x) = typeof(x) <: Union{<: Nothing}; test(12) # crash
Unreachable reached at 0x7fad662de155

Can someone explain this behavior? The fact it cause a proper crash C-side makes me suspicious that this may be a bug in the C API.

[martinholters]

Simplified: (() -> Union{<:Nothing})(). Fix in progress.