internal/encoding/yaml: support alias to scalars as map keys

This makes the decoder follow aliases in YAML map keys.
Before this commit the decoder would outright reject these keys as
non-scalar keys. This commit adds support specifically for keys which
are aliases to scalar values. YAML also permits non-scalar keys such as
maps / sequences, but there's no equivalent for those in CUE, so we
error out.

Note that this used to work in CUE v0.8.2, before we transitioned
from our own older fork of go-yaml to yaml.v3. The minor regression in
behavior was unintentional and went unnoticed as we lacked test cases.

Fixes #3821.

Closes #3822 as merged as of commit b6c33cd.

Signed-off-by: Omri Steiner <[email protected]>
Change-Id: I53bfc09a40ab0a1bbfabde63db1690dbb9222584
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1213252
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: OmriSteiner

internal/encoding/yaml/decode.go

@@ -390,9 +390,6 @@ outer:
 			}
 			continue
 		}
-		if yk.Kind != yaml.ScalarNode {
-			return d.posErrorf(yn, "invalid map key: %v", yk.ShortTag())
-		}
 
 		field := &ast.Field{}
 		label, err := d.label(yk)
@@ -450,17 +447,33 @@ func (d *decoder) merge(yn *yaml.Node, m *ast.StructLit, multiline bool) error {
 func (d *decoder) label(yn *yaml.Node) (ast.Label, error) {
 	pos := d.pos(yn)
 
-	expr, err := d.scalar(yn)
+	var expr ast.Expr
+	var err error
+	var value string
+	switch yn.Kind {
+	case yaml.ScalarNode:
+		expr, err = d.scalar(yn)
+		value = yn.Value
+	case yaml.AliasNode:
+		if yn.Alias.Kind != yaml.ScalarNode {
+			return nil, d.posErrorf(yn, "invalid map key: %v", yn.Alias.ShortTag())
+		}
+		expr, err = d.alias(yn)
+		value = yn.Alias.Value
+	default:
+		return nil, d.posErrorf(yn, "invalid map key: %v", yn.ShortTag())
+	}
 	if err != nil {
 		return nil, err
 	}
+
 	switch expr := expr.(type) {
 	case *ast.BasicLit:
 		if expr.Kind == token.STRING {
-			if ast.IsValidIdent(yn.Value) && !internal.IsDefOrHidden(yn.Value) {
+			if ast.IsValidIdent(value) && !internal.IsDefOrHidden(value) {
 				return &ast.Ident{
 					NamePos: pos,
-					Name:    yn.Value,
+					Name:    value,
 				}, nil
 			}
 			ast.SetPos(expr, pos)
@@ -474,7 +487,7 @@ func (d *decoder) label(yn *yaml.Node) (ast.Label, error) {
 		}, nil
 
 	default:
-		return nil, d.posErrorf(yn, "invalid label "+yn.Value)
+		return nil, d.posErrorf(yn, "invalid label "+value)
 	}
 }
 

internal/encoding/yaml/decode_test.go

@@ -488,6 +488,12 @@ b: {
 		"a: &a [1, 2]\nb: *a",
 		"a: [1, 2]\nb: [1, 2]", // TODO: a: [1, 2], b: a
 	},
+	{
+		`a: &a "b"
+*a : "c"`,
+		`a: "b"
+b: "c"`,
+	},
 
 	{
 		"foo: ''",
@@ -921,6 +927,8 @@ var unmarshalErrorTests = []struct {
 	{"a:\n- b: *,", "test.yaml:2: did not find expected alphabetic or numeric character"},
 	{"a: *b\n", "test.yaml: unknown anchor 'b' referenced"},
 	{"a: &a\n  b: *a\n", `test.yaml:2: anchor "a" value contains itself`},
+	{"a: &a { b: c }\n*a : foo", "test.yaml:2: invalid map key: !!map"},
+	{"a: &a [b]\n*a : foo", "test.yaml:2: invalid map key: !!seq"},
 	{"value: -", "test.yaml: block sequence entries are not allowed in this context"},
 	{"a: !!binary ==", "test.yaml:1: !!binary value contains invalid base64 data"},
 	{"{[.]}", `test.yaml:1: invalid map key: !!seq`},