internal/core/adt: fix missing dereference for equal

Now we have structure sharing, we need to ensure that
Vertex values are dereferenced properly.

Issue #3060
Issue #2884
Issue #2854

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ied35aa7f7f08205bbdb54415d78e78f1b83c8b62
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194081
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/types_test.go

@@ -2345,8 +2345,6 @@ func TestEquals(t *testing.T) {
 	}}
 	for _, tc := range testCases {
 		runMatrix(t, "", func(t *testing.T, cfg *evalConfig) {
-			TODO_Sharing(t, cfg)
-
 			r := cfg.runtime()
 
 			a, err := r.Compile("a", tc.a)

internal/core/adt/equality.go

@@ -42,12 +42,24 @@ func equalVertex(ctx *OpContext, x *Vertex, v Value, flags Flag) bool {
 	if !ok {
 		return false
 	}
-	if x == y {
-		return true
-	}
+
+	// Note that the arc type of an originating node may be different than
+	// the one we are sharing. So do this check before dereferencing.
+	// For instance:
+	//
+	//    a?: #B  // ArcOptional
+	//    #B: {}  // ArcMember
 	if x.ArcType != y.ArcType {
 		return false
 	}
+
+	x = x.Indirect()
+	y = y.Indirect()
+
+	if x == y {
+		return true
+	}
+
 	xk := x.Kind()
 	yk := y.Kind()