i think below may help u understand why .
6.3.1 Arithmetic operands
6.3.1.1 Boolean, characters, and integers
1 Every integer type has an integer conversion rank defined as follows:
- No two signed integer types shall have the same rank, even if they
hav e the same
representation.
- The rank of a signed integer type shall be greater than the rank of
any signed integer
type with less precision.
- The rank of long long int shall be greater than the rank of long
int, which
shall be greater than the rank of int, which shall be greater than the
rank of short
int, which shall be greater than the rank of signed char.
- The rank of any unsigned integer type shall equal the rank of the
corresponding
signed integer type, if any.
- The rank of any standard integer type shall be greater than the
rank of any extended
integer type with the same width.
- The rank of char shall equal the rank of signed char and unsigned
char.
- The rank of _Bool shall be less than the rank of all other standard
integer types.
- The rank of any enumerated type shall equal the rank of the
compatible integer type
(see 6.7.2.2).
- The rank of any extended signed integer type relative to another
extended signed
integer type with the same precision is implementation-defined, but
still subject to the
other rules for determining the integer conversion rank.
- For all integer types T1, T2, and T3, if T1 has greater rank than
T2 and T2 has
greater rank than T3, then T1 has greater rank than T3.
2 The following may be used in an expression wherever an int or
unsigned int may
be used:
42 Language §6.3.1.1
©ISO/IEC ISO/IEC 9899:1999 (E)
- An object or expression with an integer type whose integer
conversion rank is less
than the rank of int and unsigned int.
- A bit-field of type _Bool, int, signed int, or unsigned int.
If an int can represent all values of the original type, the value is
converted to an int;
otherwise, it is converted to an unsigned int. These are called the
integer
promotions.48) All other types are unchanged by the integer promotions.
3 The integer promotions preserve value including sign. As discussed
earlier, whether a
''plain'' char is treated as signed is implementation-defined.
Forward references: enumeration specifiers (6.7.2.2), structure and
union specifiers
(6.7.2.1).