internal/core/adt: fix let closedness issue with API use

Code did not handle lets yet. Cross checked that other places
where IsHidden is used, IsLet is used as well if applicable.

Fixes #2325

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I774560818559143d6a02f8a3f33dff8c97485f94
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552317
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/types_test.go

@@ -2109,12 +2109,12 @@ func TestSubsumes(t *testing.T) {
 }
 
 func TestUnify(t *testing.T) {
-	a := []string{"a"}
-	b := []string{"b"}
+	a := "a"
+	b := "b"
 	testCases := []struct {
 		value string
-		pathA []string
-		pathB []string
+		pathA string
+		pathB string
 		want  string
 	}{{
 		value: `4`,
@@ -2144,12 +2144,24 @@ func TestUnify(t *testing.T) {
 		pathA: a,
 		pathB: b,
 		want:  `{"a":"foo"}`,
+	}, {
+		// Issue #2325: let should not result in a closedness error.
+		value: `#T: {
+			...
+		}
+		b: {
+			let foobar = {}
+			 _fb: foobar
+		}`,
+		pathA: "#T",
+		pathB: b,
+		want:  `{}`,
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.value, func(t *testing.T) {
 			v := getInstance(t, tc.value).Value()
-			x := v.Lookup(tc.pathA...)
-			y := v.Lookup(tc.pathB...)
+			x := v.LookupPath(ParsePath(tc.pathA))
+			y := v.LookupPath(ParsePath(tc.pathB))
 			b, err := x.Unify(y).MarshalJSON()
 			if err != nil {
 				t.Fatal(err)

internal/core/adt/composite.go

@@ -772,6 +772,11 @@ func (v *Vertex) IsClosedList() bool {
 
 // TODO: return error instead of boolean? (or at least have version that does.)
 func (v *Vertex) Accept(ctx *OpContext, f Feature) bool {
+	// TODO(v0.6): move f.IsHidden from below to here.
+	if f.IsLet() {
+		return true
+	}
+
 	if x, ok := v.BaseValue.(*Disjunction); ok {
 		for _, v := range x.Values {
 			if x, ok := v.(*Vertex); ok && x.Accept(ctx, f) {