func deref(x ast.Expr) ast.Expr {
	if p, isPtr := x.(*ast.StarExpr); isPtr {
		x = p.X
	}
	return x
}

// checkExprOrType checks that x is an expression or a type
// (and not a raw type such as [...]T).
func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
	switch t := ast.Unparen(x).(type) {
	case *ast.ParenExpr:
		panic("unreachable")
	case *ast.UnaryExpr: