unreachable reached when struct constructor trys to convert a Vector{Union{Nothing, T} where T} to a Vector{Any}

[Roger-luo]

MWE:

struct Foo
   a::Vector{Any}
end

xs = Vector{Union{Nothing, T} where T}([2, 3])
Foo(xs)

this leads to "illegal hardware instruction" on both Julia 1.5.2 and julia 1.6.0-DEV.1184

The lowered code and typed code seems fine, but code_llvm gives

julia> @code_llvm Foo(xs)
;  @ REPL[1]:2 within `Foo'
define [1 x {}*] @julia_Foo_491({}* nonnull align 16 dereferenceable(40) %0) {
top:
  call void @llvm.trap()
  unreachable
}

also I find that the result of convert and Vector{Any}(x) does not match, though convert actually (should) calls Vector{Any} to convert xs to Vector{Any} (https://github.com/JuliaLang/julia/blob/master/base/array.jl#L554), e.g

julia> convert(Vector{Any}, xs)
2-element Vector{Union{Nothing, T} where T}:
 2
 3

julia> Vector{Any}(xs)
2-element Vector{Any}:
 2
 3

This is because Julia thinks xs of type Vector{Union{Nothing, T} where T} is a Vector{Any}, so convert returns xs directly but I'm not sure if this is actually true...

[vtjnash]

It is trivially true:

julia> Any == Union{Nothing, T} where T
true

But datatype creation is not handling this

[vtjnash]

Duplicate of #35130