julia crashes during method disptch

[OkonSamuel]

recently julia dispatch used a less specific method rather than the expected more specific method. While playing around i discovered that following code crashes julia.

julia> using SentinelArrays
b 
julia> b =SentinelArray([1,2,3,4])
4-element SentinelArray{Int64,1,Int64,Missing,Array{Int64,1}}:
 1
 2
 3
 4

julia> f(::Type{T}, y, x::AbstractVector{T}) where T = (T, y)
f (generic function with 1 method)

julia> function g(x, y)
       if x == Int64
        z = true
       else
        z= false
        end
        return f(x, z, y)
        end
g (generic function with 1 method)

julia> f(Union{Missing, Int64}, true, b)
(Union{Missing, Int64}, true)

julia> Union{Missing, Int64} == Int64
false

julia> f(Union{Missing, Int64}, true, b)
(Union{Missing, Int64}, true)

julia> f(Union{Missing, Int64}, false, b)
(Union{Missing, Int64}, false)

julia> g(Union{Missing, Int64}, b)
Unreachable reached at 0x7ff87d4a2085signal (4): Illegal instruction
in expression starting at REPL[9]:1
g at ./REPL[4]:7
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2159 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2323
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1700 [inlined]
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:369
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:458
eval_stmt_value at /buildworker/worker/package_linux64/build/src/interpreter.c:409 [inlined]
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:817
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:911
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:819
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:769
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/toplevel.c:848
eval at ./boot.jl:331
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2145 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2323
eval_user_input at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:118 [inlined]
#26 at ./task.jl:358
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2145 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2323
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1700 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:687
unknown function (ip: (nil))
Allocations: 30348217 (Pool: 30340443; Big: 7774); GC: 28
Illegal instruction (core dumped)

is this behavior expected??

julia> versioninfo()
Julia Version 1.4.2
Commit 44fa15b150* (2020-05-23 18:35 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i3-5005U CPU @ 2.00GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-8.0.1 (ORCJIT, broadwell)

[Keno]

Inference issue:

julia> code_typed(g, Tuple{Union, typeof(b)})
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = (x == Main.Int64)::Bool
└──      goto #3 if not %1
2 ─      goto #4
3 ─      nothing::Nothing
4 ┄ %5 = φ (#2 => true, #3 => false)::Bool
│        Main.f(x, %5, y)::Union{}
└──      unreachable
) => Union{}

julia> code_typed(g, Tuple{Type{Union{Missing, Int}}, typeof(b)})
1-element Vector{Any}:
 CodeInfo(
1 ─ %1  = Main.Int64::Core.Const(Int64, false)
│   %2  = $(Expr(:foreigncall, :(:jl_types_equal), Int32, svec(Any, Any), 0, :(:ccall), Core.Argument(2), :(%1)))::Int32
│   %3  = Core.sext_int(Core.Int64, %2)::Int64
│   %4  = (%3 === 0)::Bool
│   %5  = Base.not_int(%4)::Bool
└──       goto #3 if not %5
2 ─       goto #4
3 ─       nothing::Nothing
4 ┄ %9  = φ (#2 => true, #3 => false)::Bool
│   %10 = Core.tuple(Union{Missing, Int64}, %9)::Tuple{Union,Bool}
└──       return %10
) => Tuple{Union,Bool}

julia> Type{Union{Missing, Int}} <: Union
true

[Keno]

Actually a type intersection issue. Reduced:

julia> struct F{T, V} <: AbstractArray{Union{T, V}, 1}
       end
julia> A = Tuple{Type{T}, AbstractVector{T}} where T
Tuple{Type{T},AbstractVector{T}} where T

julia> B = Tuple{Union, F{Int64, Missing}}
Tuple{Union,F{Int64,Missing}}

julia> typeintersect(A, B) <: A
false

julia> typeintersect(A, B) <: B
false

[vtjnash]

Those should be >: (though they still give false). The error being in the first parameter to F, which has managed to become not-equal to the first parameter from B.

julia> typeintersect(A, B)
Tuple{Union, F{Union{Missing, Int64}, Missing}}