Require every choice variant to carry a path, and list only what Fake accepts

render.go

@@ -28,9 +28,11 @@ func (f *Fakes) Fake(path string) (string, error) {
 	return render(f.rand, n), nil
 }
 
-// descend walks named fields, resolving choices it meets along the way. It is
-// the one render-side step that can fail, because the path comes from the
-// caller and may name a field that does not exist.
+// descend walks named fields to the node a path names. It is the one render-side
+// step that can fail, because the path comes from the caller and may name a field
+// that does not exist. A choice consumes no segment, so every variant must carry
+// the rest of the path before one is picked at random — a path that resolves at
+// all resolves on every call. A nil session validates without picking.
 func descend(s *session, n node, segments []string) (node, error) {
 	if len(segments) == 0 {
 		return n, nil
@@ -49,7 +51,22 @@ func descend(s *session, n node, segments []string) (node, error) {
 		}
 		return descend(s, child, segments[1:])
 	case *choice:
-		return descend(s, pick(s, n), segments) // a choice consumes no path segment
+		for i, item := range n.items {
+			_, err := descend(nil, item, segments)
+			if err == nil {
+				continue
+			}
+			if len(n.items) == 1 { // a single-variant choice is a transparent wrapper
+				return nil, err
+			}
+			return nil, fmt.Errorf("variant %d of a %d-way choice: %w", i+1, len(n.items), err)
+		}
+		if s == nil {
+			return n, nil
+		}
+		return descend(s, pick(s, n), segments)
+	case literal:
+		return nil, fmt.Errorf("a plain string has no field %q", segments[0])
 	default:
 		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
 	}