parameter hoisting changes program behaviour

[zerbina]

The parameter hoisting necessary for default parameter expressions referencing other parameters changes the evaluation order of the call arguments.

Example

proc a(x: int, y: int, z = y + 1) =
  discard

proc b(x: int, y: int, z = y + x) = # the order of usage is significant!
  discard

a((echo "a"; 1), (echo "b"; 2)) # 1
b((echo "c"; 1), (echo "d"; 2)) # 2

Actual Output

b
a
d
c

Expected Output

a
b
c
d

Additional Information

The underlying issue is with how the parameter hoisting commits the argument expression for the hoisted parameters to temporaries:

call(x = a, y = b, z = y + 1) # `y` needs to be hoisted

# is transformed into

let
  :tmp = b
call(a, :tmp, :tmp + 1)

If a has side-effects, the transformation changes the program's meaning (case number 1).

In case number 2, both parameter are hoisted, but in the order of in which they're used and not in the order in which the parameter are specified.

To preserve the meaning of the program, if a parameter is hoisted, all preceding parameters need to be hoisted too. In addition, parameters always need to be hoisted from left to right.