7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.20 Comma operator [expr.comma]

The comma operator groups left-to-right.
A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression.
The left expression is sequenced before the right expression ([intro.execution]).
The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a bit-field.
[Note 1:
In contexts where the comma token is given special meaning (e.g. function calls ([expr.call]), subscript expressions ([expr.sub]), lists of initializers ([dcl.init]), or template-argument-lists ([temp.names])), the comma operator as described in this subclause can appear only in parentheses.
[Example 1:
f(a, (t=3, t+2), c); has three arguments, the second of which has the value 5.
— end example]
— end note]