Inlined dispatch uses incorrect (type-equivalent, but not egal) type

[topolarity]

is_union(::Type{T}) where T = "not Union!"
is_union(T::Union) = "Union!"

@inline f(i, nm, T) = (println(T); is_union(T))

applyfield(f, ::Type{T}, i) where T = f(i, fieldname(T, i)::Symbol, fieldtype(T, i))

struct Foo{I<:Union{Nothing,<:Number}} value::I end
struct Bar{I<:Union{Nothing,Number}} value::I end

The dispatch in f ends up "sticky":

julia> applyfield(f, Foo, 1)
Union{Nothing, var"#s1"} where var"#s1"<:Number
"not Union!"

julia> applyfield(f, Bar, 1)
Union{Nothing, Number}
"not Union!"

The dispatch in f ends up evaluated with a different (type-equivalent, but not egal) T.

The opposite results happen if you do Bar first then Foo.

[topolarity]

Reduced from a JSON3.jl example that exhibits similar "stickiness":

using JSON3

struct Foo{I<:Union{Nothing,<:Number}}
  name::String
  value::I
end

serialized_ = JSON3.write(Foo("test", nothing))
deserialized_ = JSON3.read(serialized_, Foo) # if this runs first, both `read`s fail

struct Bar{I<:Union{Nothing,Number}}
  name::String
  value::I
end

serialized_ = JSON3.write(Bar("test", nothing))
deserialized_ = JSON3.read(serialized_, Bar) # if this runs first, both `read`s succeed

[mikmoore]

A recent comment and the surrounding discussion seems somewhat related to the complicated and brittle handling of Union. I didn't really understand the problem or explanation, however, so I can't say anything smart here.