[conv.prom]

A prvalue that is not a converted bit-field and has an integer type other than bool, char8_t, char16_t, char32_t, or wchar_t whose integer conversion rank ([conv.rank]) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.

A prvalue of an unscoped enumeration type whose underlying type is not fixed can be converted to a prvalue of the first of the following types that can represent all the values of the enumeration ([dcl.enum]): int, unsigned int, long int, unsigned long int, long long int, or unsigned long long int.

If none of the types in that list can represent all the values of the enumeration, a prvalue of an unscoped enumeration type can be converted to a prvalue of the extended integer type with lowest integer conversion rank ([conv.rank]) greater than the rank of long long in which all the values of the enumeration can be represented.

If there are two such extended types, the signed one is chosen.

A prvalue of an unscoped enumeration type whose underlying type is fixed ([dcl.enum]) can be converted to a prvalue of its underlying type.

Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type.

[Note 1:

A converted bit-field of enumeration type is treated as any other value of that type for promotion purposes.

— end note]

A converted bit-field of integral type can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field.

A prvalue of type char8_t, char16_t, char32_t, or wchar_t ([basic.fundamental]) (including a converted bit-field that was not already promoted to int or unsigned int according to the rules above) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long int, unsigned long int, long long int, unsigned long long int, or its underlying type.

A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.

These conversions are called integral promotions.