Fix ambiguous flow style yaml

syntax/encoding/yaml.go

@@ -629,8 +629,34 @@ func DecodeYAML(filename string, d *yaml.Decoder, tags TagDecoder) (syntax.Node,
 // encoding process.
 func EncodeYAML(e *yaml.Encoder, n syntax.Node) syntax.Diagnostics {
 	yamlNode, diags := MarshalYAML(n)
+	fixFlowStyles(yamlNode)
+
 	if err := e.Encode(yamlNode); err != nil {
 		diags.Extend(syntax.Error(nil, err.Error(), ""))
 	}
 	return diags
 }
+
+// fixFlowStyles converts flow-style mappings/sequences to block style when they contain comments,
+// which can cause ambiguous YAML that stricter parsers reject.
+func fixFlowStyles(node *yaml.Node) {
+	if node == nil {
+		return
+	}
+	if (node.Kind == yaml.MappingNode || node.Kind == yaml.SequenceNode) &&
+		node.Style&yaml.FlowStyle != 0 && hasChildWithComment(node) {
+		node.Style = 0
+	}
+	for _, child := range node.Content {
+		fixFlowStyles(child)
+	}
+}
+
+func hasChildWithComment(node *yaml.Node) bool {
+	for _, child := range node.Content {
+		if child.LineComment != "" || child.HeadComment != "" || child.FootComment != "" {
+			return true
+		}
+	}
+	return false
+}

syntax/encoding/yaml_test.go

@@ -179,6 +179,34 @@ baz: qux
 	assert.Equal(t, expected, b.String())
 }
 
+func TestYAMLFlowWithChildCommentReEmitsAsBlock(t *testing.T) {
+	// A flow-style sequence whose child carries a comment produces ambiguous YAML
+	// when re-emitted as flow (e.g. `items: [one, # first\n  two, three]`), which
+	// stricter parsers reject. EncodeYAML should convert such nodes to block style.
+	const doc = `items: [
+  one, # first
+  two,
+  three,
+]
+`
+	const expected = `items:
+  - one # first
+  - two
+  - three
+`
+
+	rootNode, diags := DecodeYAML("yaml", yaml.NewDecoder(strings.NewReader(doc)), nil)
+	require.Empty(t, diags)
+
+	var b bytes.Buffer
+	enc := yaml.NewEncoder(&b)
+	enc.SetIndent(2)
+	diags = EncodeYAML(enc, rootNode)
+
+	assert.Empty(t, diags)
+	assert.Equal(t, expected, b.String())
+}
+
 func TestYAMLDeleteAllValuesThenAdd(t *testing.T) {
 	const doc = `values:
     example1: abc`