"ref" and "term" for the non-operator kinds
<code>expr_kind::expr_ref</code> amd <code>expr_kind::terminal</code>,
respectively.*/
- inline char const * op_string(expr_kind kind)
+ inline constexpr char const * op_string(expr_kind kind)
{
switch (kind) {
case expr_kind::expr_ref: return "ref";