Reject incomplete defaults

[sdboyer]

With the model we're pursuing for generating defaults, it doesn't make sense to allow non-concrete default values. That means any of these cases should result in an error:

A: int | *string
B: "foo" | *("bar" | "baz")
C: "foo" | "bar" | *_|_

Not sure how realistic/important C is, but the other two definitely are.

[ying-jeanne]

currently when we import case C into cue, c.Default() would return false with value |.

for a normal case when cue.Value doesn't have default, the function Default() return also false with value |. we need to think if we want to manage case C, then a walk around would be needed :).

[sdboyer]

Interestingly, i now know that this dovetails with another subtle challenge - the v.Default() where v.IncompleteKind() is cue.List is almost always itself. That means that determining whether a list instance actually has a default value is largely about checking whether the default differs from the input.

This is in contrast to every other Kind of value. Lists are weird.