GATs allow dispatching on types. Below theory T2 adds a new operation with the name h:

@theory T1 begin 
  X::TYPE; h(a::X)::X 
end
@theory T2<:T1 begin 
  Y::TYPE; h(b::Y)::Y 
end

@instance T1{Int} begin h(a::Int) = 1 end

There is a problem when trying to implement an instance of T2.

# ERROR # 
@instance T2{Int,Bool} begin 
  @import h
  h(b::Bool) = false 
end

The @import keyword within the @instance macro is commonly used when something has been implemented already. However, in this case, we cannot @import h because then h(::Bool) is not one of the expected signatures and it errors upon seeing that line.

I'll upload a fix to this soon that is working locally: basically it changes what @import does within the instance macro. Rather than removing expected signatures, whenever we have leftover unimplemented signatures (such as h(::Int) in the case above) we check if the name of the method is in the imports and ignore the missing method error if so.