add x, y =
    x + y

print! add 1, 2
print! add 1, "a" # ERR
print! add "a", 1 # ERR

add_l x: Int, y = x + y
print! add_l 1, 2
print! add_l 1, "a" # ERR
print! add_l "a", 1 # ERR

add_r x, y: Int = x + y
print! add_r 1, 2
print! add_r 1, "a" # ERR
print! add_r "a", 1 # ERR

invalid_add x: Int, y: Str = x + y # ERR

C = Class()
C|C <: Add(C)|.
    Output = C
    __add__ self, other = self
D = Class()
D|D <: Add(C)|.
    Output = D
    __add__ self, other = self

same_add: |T <: Add(T)|(x: T, y: T) -> T.Output
same_add x, y = x + y

print! add(D.new(), C.new())
print! add(C.new(), D.new()) # ERR
print! same_add(1, 2.1)
print! same_add(C.new(), C.new())
print! same_add(D.new(), C.new()) # ERR