internal/core/adt: use partial lookup

For selectors and indices, don't pass Finalized state
for selector operand. Currently this doesn't matter,
but in the comprehension changes such nodes are more
actively evaluated, which may lead to spurious cycles.

Not doing this active evaluation, in turn, may lead
to void arcs being detected as full arcs.

Change-Id: Iba539a379bd9e68cd7a3b70e5d249476dcfc0698
Signed-off-by: Marcel van Lohuizen <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/533293
Unity-Result: CUEcueckoo <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>

internal/core/adt/context.go

@@ -733,7 +733,7 @@ func (c *OpContext) unifyNode(v Expr, state VertexStatus) (result Value) {
 			return nil
 		}
 
-		if v.isUndefined() {
+		if v.isUndefined() || state > v.status {
 			// Keep a minimum state of AllArcs.
 			state := state
 			if state < AllArcs {
@@ -844,6 +844,11 @@ outer:
 		}
 		x.state.usedArcs = append(x.state.usedArcs, a)
 	}
+
+	if a != nil && state > a.status {
+		c.Unify(a, state)
+	}
+
 	if a == nil {
 		if x.state != nil {
 			for _, e := range x.state.exprs {

internal/core/adt/expr.go

@@ -897,7 +897,11 @@ func (x *SelectorExpr) Source() ast.Node {
 }
 
 func (x *SelectorExpr) resolve(c *OpContext, state VertexStatus) *Vertex {
-	n := c.node(x, x.X, x.Sel.IsRegular(), state)
+	// TODO: the node should really be evaluated as AllArcs, but the order
+	// of evaluation is slightly off, causing too much to be evaluated.
+	// This may especially result in incorrect results when using embedded
+	// scalars.
+	n := c.node(x, x.X, x.Sel.IsRegular(), Partial)
 	if n == emptyNode {
 		return n
 	}
@@ -929,7 +933,11 @@ func (x *IndexExpr) Source() ast.Node {
 
 func (x *IndexExpr) resolve(ctx *OpContext, state VertexStatus) *Vertex {
 	// TODO: support byte index.
-	n := ctx.node(x, x.X, true, state)
+	// TODO: the node should really be evaluated as AllArcs, but the order
+	// of evaluation is slightly off, causing too much to be evaluated.
+	// This may especially result in incorrect results when using embedded
+	// scalars.
+	n := ctx.node(x, x.X, true, Partial)
 	i := ctx.value(x.Index)
 	if n == emptyNode {
 		return n