Type cast failed in switch for enum with restricted storage

SSCCE:

enum class confirm {yes};

struct item
{
  confirm s:4; // (1) limiting storage size required
};

int main()
{
  item itm;

  itm.s = confirm::yes; // (2) OK

  switch (itm.s)
  {
    case confirm::yes: // (3) Failure, need static data cast here?
      break;
  }
}

produces error:

In function ‘int main()’:
error: could not convert ‘yes’ from ‘confirm’ to ‘int’
 case confirm::yes:
               ^

while compiling with g++ but compiled fine by clang++. Why assignment marked by (2) possible but case clause marked by (3) not?

Warning about too small storage is of offtopic