Rule 10.3 - Bitfield narrower essential type

[apereira]

Hello,

Assuming that I have the following structure and enum:

Code:

typedef struct s{
  unsigned int a:5;
  unsigned int b:1;
  unsigned int c:1;
} s;

typedef enum e{
  ZERO = 0,
  ONE = 1,
  TWO = 2,
  THREE = 3
} e;

Why does the following code trigger rule 10.3?

Code:

s fcn(void)
{
  s test;

  test.a = (unsigned int) THREE;

  return test;
}