This is the problem: => is actually just Pair, a type, so typeof just returns DataType.
julia> Core.Compiler.return_type(f, Tuple{Type{Pair},Int,Int})
Pair{Int64, Int64}
This is the problem: => is actually just Pair, a type, so typeof just returns DataType.
julia> Core.Compiler.return_type(f, Tuple{Type{Pair},Int,Int})
Pair{Int64, Int64}