Document loss of default values for method parameters when doubling

[bitwes]

Godot does not provide metadata about default parameters yet. Since it can't be known which parameters are defaulted, GUT defaults ALL parameters to ALL methods to null. This can cause hard to figure out issues when you do a partial_double (or stub a method to_call_super) and a method has a parameter that is defaulted to something other than null.

Example:

var _sum  = 0
func increment(inc_by=1):
  _sum += inc_by

The double definition for increment will be:

func increment(inc_by=null):

If this method is part of a partial_double or is stubbed to_call_super and is called without parameters during a test (directly or indirectly) then this will cause an Invalid operands 'int' and 'Nil' error.

[astrale-sharp]

Hey, so what would be the plan to tackle this problem? (waiting metadata and using them?) :)

[bitwes]

There is a Godot issue for this: godotengine/godot#33624

I think I'm going to expand on the solution for #266 and allow you to specify default values in addition to the number of parameters.

[bitwes]

Introduced stub(...).param_defaults([]) which allows you to specify the default values that a method should get.

[bitwes]

7.2.0 you can now stub default values for method parameters:

var Foo = load('res://foo.gd')
test_go_up_one_increments_sum_by_1():
  var dbl_foo = double(Foo).new()
  stub(dbl_foo, 'go_up_one').to_call_super().param_defaults([1])

  dbl_foo.go_up_one()
  assert_called(dbl_foo, 'increment', [1])