If I create a new function f(x::Float64) and you call it like f(1), it will fail with a MethodError:
julia> f(x::Float64) = x + 1.0
f (generic function with 1 method)
julia> f(1)
ERROR: MethodError: no method matching f(::Int64)
This is exactly what’s happening in your example: there’s no method for copy that applies in your example. You might argue that (a) Julia should have a definition for all possible types or that (b) Julia should at least have definitions for “core” types like Tuple. I’m sympathetic to (a), but the general feeling is that Julia don’t do this. I fully agree with (b) and think submitting a PR that adds copy for Tuple is useful. You might find the current committers disagree, but that presumably depends on them disagreeing with (b).