Argument order dependency in dispatching on Type{Union{T,S}} and Type{T}

[tkf]

Originally discussed in: https://discourse.julialang.org/t/19732/5

Following "poor man's typesubtract" does not work:

julia> f(::Type{Union{T,S}}, ::Type{T}) where {S, T} = S
f (generic function with 1 method)

julia> f(Union{Missing, Float64, Int}, Missing)
ERROR: MethodError: no method matching f(::Type{Union{Missing, Float64, Int64}}, ::Type{Missing})
Closest candidates are:
  f(::Type{Union{T, S}}, ::Type{T}) where {S, T} at REPL[2]:1
Stacktrace:
 [1] top-level scope at REPL[3]:1

However, it works when the order of arguments is reversed:

julia> g(::Type{T}, ::Type{Union{T,S}}) where {S, T} = S
g (generic function with 1 method)

julia> g(Missing, Union{Missing, Float64, Int})
Union{Float64, Int64}

I checked it with 1.2.0-DEV.146. I also noticed that both of them work with 0.6.4. This could be a duplicate of #26135 and #24333. But since none of them mention the order of arguments, I post it just in case.