Core._apply segfaults on svec(#undef)

[vtjnash]

Is svec(#undef) supposed to be visible in Julia? Right now we have a case where typeintersect might fail to fill in type-parameters, leading to a segfault when we splat the result into an apply_type call:

julia> const RO38{B<:Val, M<:AbstractMatrix} =
           Tuple{
           <:Union{B, <:Val{<:B}},
           M,
           Union{AbstractMatrix{B}, AbstractMatrix{<:Vector{<:B}}}}
Tuple{var"#s1", M, Union{AbstractMatrix{B}, AbstractMatrix{<:Vector{<:B}}}} where {B<:Val, M<:(AbstractMatrix), var"#s1"<:(Union{B, var"#s2"} where var"#s2"<:(Val{<:B}))}

julia> typeintersect(RO38{T,S} where {T,S}, RO38)
Tuple{Union{var"#s1", var"#s11"}, 
[3247358] signal (11): Segmentation fault
in expression starting at none:0
ijl_isa at /data/vtjnash/julia/src/subtype.c:2075
sig_match_simple at /data/vtjnash/julia/src/typemap.c:239 [inlined]
...
Segmentation fault

with additional MWEs:

julia> T = ccall(:jl_new_structv, Any, (Any, Ptr{Cvoid}, UInt32), Union, [S, S], 2) where S<:AbstractMatrix;

julia> (ti, env) = ccall(:jl_type_intersection_with_env, Any, (Any, Any), T, AbstractMatrix); env
svec(#undef)

julia> identity(env...)
Segmentation fault

Originally posted by @vtjnash in #46735 (comment)