tree 6deb9e2100ad775e8e0ca3bd62e74f77f662df87
parent 77b475f8a88568ae0e40a0b9ec5a2858e4545b3d
author Marcel van Lohuizen <mpvl@golang.org> 1613314556 +0100
committer Marcel van Lohuizen <mpvl@golang.org> 1613385962 +0000

internal/core/adt: strip "notDefault" status for disjunctions with unused defaults

This most fixes all known default issues. The semantics is still not
entirely right, and will require a slightly different order of evaluation for
disjunctions. This is best addressed when doing some of the planned
performance enhancements for disjunctions.

Fixes #763
Fixes #726

This CL changes the semantics as follows:
1) a marked disjunction becomes an unmarked disjunction if,
after the conjuncts are distributed over disjuncts, none of
its marked disjuncts remain
2) a nested disjunction a | b | (nestedA | nestedC) is
evaluated according to the same rules, and then incorporated as is.

This implementation still slightly deviates from these rules:
The probem with the current implementation is that nested
disjunctions are unified agaist existing conjuncts, and possibly
eliminated early, before applying the rules. So in
```
     ("a" | "b") & (*(*"a" | string) | string)
```
the inner `*"a" | string` is combined with `"a" | "b"` before it
is determined whether its result is a marked disjunction. This was
okay with the previous semantics and had the benefit of allowing to
eliminate some disjuncts earlier, but no longer works with the new
semantics. Also if we use structure sharing, the benefits of reuse
will likely be bigger then early elimination in this case.

Some of the complexity of the logic in this CL is introduced
to deal with this discrepency. It can be simplified again
considerably when the required change in control flow is
implemented.

Change-Id: I638bbb66ba69bd1642261dd2628dfcc27b14f893
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8706
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>