[expr.arith.conv]

Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way.

The purpose is to yield a common type, which is also the type of the result.

This pattern is called the usual arithmetic conversions, which are defined as follows:

(1.1)
The lvalue-to-rvalue conversion ([conv.lval]) is applied to each operand and the resulting prvalues are used in place of the original operands for the remainder of this section.
(1.2)
If either operand is of scoped enumeration type ([dcl.enum]), no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.
(1.3)
Otherwise, if either operand is of floating-point type, the following rules are applied:
- (1.3.1)
  If both operands have the same type, no further conversion is needed.
- (1.3.2)
  Otherwise, if one of the operands is of a non-floating-point type, that operand is converted to the type of the operand with the floating-point type.
- (1.3.3)
  Otherwise, if the floating-point conversion ranks ([conv.rank]) of the types of the operands are ordered but not equal, then the operand of the type with the lesser floating-point conversion rank is converted to the type of the other operand.
- (1.3.4)
  Otherwise, if the floating-point conversion ranks of the types of the operands are equal, then the operand with the lesser floating-point conversion subrank ([conv.rank]) is converted to the type of the other operand.
- (1.3.5)
  Otherwise, the expression is ill-formed.
(1.4)
Otherwise, each operand is converted to a common type C.

The integral promotion rules ([conv.prom]) are used to determine a type T1 and type T2 for each operand.48
Then the following rules are applied to determine C:
- (1.4.1)
  If T1 and T2 are the same type, C is that type.
- (1.4.2)
  Otherwise, if T1 and T2 are both signed integer types or are both unsigned integer types, C is the type with greater rank.
- (1.4.3)
  Otherwise, let U be the unsigned integer type and S be the signed integer type.
  
  (1.4.3.1)
  If U has rank greater than or equal to the rank of S, C is U.
  (1.4.3.2)
  Otherwise, if S can represent all of the values of U, C is S.
  (1.4.3.3)
  Otherwise, C is the unsigned integer type corresponding to S.

48)48)

As a consequence, operands of type bool, char8_t, char16_t, char32_t, wchar_t, or of enumeration type are converted to some integral type.

7 Expressions [expr]

7.4 Usual arithmetic conversions [expr.arith.conv]