Wrapper{Type{Int}} really needed?

General Usage Performance
2

That’s likely just constant folding in the specific case. If you add a Wrapper argument to f, the inference fails as expected:

julia> function f(b::Wrapper)
           a = Wrapper(Int)
           promote_type(a.value, b.value)
       end
f (generic function with 2 methods)

julia> Base.promote_op(f, typeof(Wrapper(Float64)))
Type{S} where S
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