Looks like a bug and flies like a bug, is it a bug?

bug

#1
julia> struct T end

julia> (t::TT)(x) where {TT<:T} = x
ERROR: function type in method definition is not a type
Stacktrace:
 [1] top-level scope at none:0

#2

I think this is

Also, please consider choosing a more specific title for your topic.