Erratic hole type resolution
Solution 1:
If you want to generate all possible such values, then you can write a function to do so, either with provided or specified bounds.
It may very well be possible to use type-level Church Numerals or some such so as to enforce creation of these, but it's almost definitely too much work for what you probably want/need.
This might not be what you want (i.e. "Except of using just (x, y) since z = 5 - x - y") but it makes more sense than trying to have some kind of enforced restriction on the type level for allowing valid values.