Type parameter corrupted when matching

[alyst]

This should be the more confined example of the bug that hit JuliaData/WeakRefStrings.jl#20 on Julia nightly (Julia 0.7.0-DEV.3018):

julia> foo(A::Array{T}) where {T} = @show T eltype(A)
foo (generic function with 1 method)

julia> bar(A::Array{T}) where {T <: Union{Integer, Missing}} = @show T eltype(A)
bar (generic function with 1 method)

julia> bar2(A::Array{T}) where {T <: Union{Int, Missing}} = @show T eltype(A)
bar2 (generic function with 1 method)

julia> foo([1, missing])
T = Union{Missing, Int64}
eltype(A) = Union{Missing, Int64}
Union{Missing, Int64}

julia> bar([1, missing])
T = Int64
eltype(A) = Union{Missing, Int64}
Union{Missing, Int64}

julia> bar2([1, missing])
T = Union{Missing, Int64}
eltype(A) = Union{Missing, Int64}
Union{Missing, Int64}

For some reason T===Int instead of T=Union{Int, Missing} inside bar().

[nalimilan]

Sounds similar to the bug signalled at #24653 (comment), which has a @test_broken call.

[alyst]

Actually, if Missing is replaced with Float64 and the functions are called like foo(Union{Int, Float64}[1, 1.5]) the bug is still there.