The method unit(x::Type{Union{Missing,T}}) is causing stack overflow

[SeSodesa]

I've been working on adding Unitful support to FiniteDifferences.jl in JuliaDiff/FiniteDifferences.jl#244, and ran into this issue when running the test suite of the package. The method

Unitful.jl/src/utils.jl

Lines 143 to 145 in c1a9b4e

	@inline unit(x::Type{T}) where {T <: Number} = NoUnits
	@inline unit(x::Type{Union{Missing, T}}) where T = unit(T)
	@inline unit(x::Type{Missing}) = missing

on line 144 of utils.jl is causing stack overflow, probably because of the methods immediately surrounding it. Why does this method exist, if we already have separate methods for Missing and T <: Any? To prefer T != Missing, if it exists?

Anyways, here is the total output of the FiniteDifferences.jl test suite for reference: test.out.txt.

[SeSodesa]

If I am not mistaken, an f::Function might get passed in as an argument to one of the FiniteDifferences.jl functions, and the updated code attempts to observe its unit. I guess this case is not handled by Unitful code yet.

[sostock]

My guess is that this method exists because unit may be called on the eltype of some container, and that eltype may be Union{T,Missing}.

[SeSodesa]

Ah, that might be it. But anyways, I might have a solution soon that at least fixes the tests in the FiniteDifferences package. Whether it is acceptable here is another question.