Missing implict conversion from serializable enum to underlying type when used as case label

[apinski-cavium]

Take:

enum bit<4> e
{
  a = 1
}
control c(in bit<4> v)
{
  apply{
    switch(v) {
      e.a:
      default: {}
    }
  }
}

I would had assumed there was an implict conversion from the serializable enum to the underlying type here (as so many other places do the implict conversion) but p4c rejects this code with the following error message:

testsuite/p4c.language/valid/control/bitswitch-4.p4(8): [--Werror=type-error] error: SwitchStatement: not defined on bit<4> and e
    switch(v) {
    ^^^^^^

Which is not helpful as it does not show the case label on where the problem is located.