cue: fix crash on alias use in lists

Change-Id: Ied56dd7c1c971f6ad9fcb8bec2bca28075481521
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4022
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/ast.go

@@ -297,8 +297,15 @@ func (v *astVisitor) walk(astNode ast.Node) (ret value) {
 
 		arcs := []arc{}
 		for i, e := range elts {
+			elem := v1.walk(e)
+			if elem == nil {
+				// TODO: it would be consistent to allow aliasing in lists
+				// as well, with a similar meaning as alias declarations in
+				// structs.
+				return v.errf(n, "alias not allowed in list")
+			}
 			v1.sel = strconv.Itoa(i)
-			arcs = append(arcs, arc{feature: label(i), v: v1.walk(e)})
+			arcs = append(arcs, arc{feature: label(i), v: elem})
 		}
 		s := &structLit{baseValue: newExpr(n), arcs: arcs}
 		list := &list{baseValue: newExpr(n), elem: s}

cue/ast_test.go

@@ -370,6 +370,14 @@ b: preference mark not allowed at this position:
 		out: `<0>{` +
 			`def :: (<1>C{Size: int, Type: string, Text: string} & (<2>C{Size: 0, Type: "B"} | <3>C{Size: 1, Type: "A"}))` +
 			`}`,
+	}, {
+		// Issue #172
+		in: `
+		package testenv
+		env_:: [NAME=_]: [VALUE=_]
+		env_:: foo: "bar"
+			`,
+		out: "env_.*: alias not allowed in list:\n    test:3:20\n<0>{}",
 	}}
 	for _, tc := range testCases {
 		t.Run("", func(t *testing.T) {