internal/core/adt: prevent state erasure

This change avoids marking an arc with too high
a state prematurely.

This is currently not an issue, but will be an issue
after the comprehension rework. Isolating this change
within a separate CL will help identify unforeseen
breakages that are specifically related to this change.

Also removed XXX that is now fixed.

Change-Id: Id3194aeebd905474adedd3245f15d9cb816c5865
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/529522
Reviewed-by: Paul Jolly <paul@myitcv.io>

internal/core/adt/eval.go

@@ -1391,7 +1391,7 @@ func (n *nodeContext) addVertexConjuncts(env *Environment, closeInfo CloseInfo,
 		// is necessary to prevent lookups in unevaluated structs.
 		// TODO(cycles): this can probably most easily be fixed with a
 		// having a more recursive implementation.
-		n.ctx.Unify(arc, AllArcs)
+		n.ctx.Unify(arc, Partial)
 	}
 
 	for _, c := range arc.Conjuncts {

internal/core/export/testdata/adt.txtar

@@ -8,7 +8,6 @@ p1: [X=string]: name: X
 d1: "foo\(bar)": int
 bar: "bar"
 
-// XXX: reference not resolving.
 d2: C="foo\(bar)": {
     name: "xx"
     foo: C.name
@@ -227,8 +226,6 @@ errorListDef: {
 [d1 foobar]
 [bar]
 [d2]
-- XXX: reference not resolving.
-
 [d2 foobar]
 [d2 foobar name]
 [d2 foobar foo]
@@ -309,8 +306,6 @@ _|_ // e3: index out of range [2] with length 2
 		foobar: int
 	}
 	bar: "bar"
-
-	// XXX: reference not resolving.
 	d2: {
 		foobar: {
 			name: "xx"