cpp/language/attributes/assume

Specifies that an expression will always evaluate to at a given point.

Explanation
Can only be applied to a null statement, as in. This statement is called an assumption. If the expression (contextually converted to ) would not evaluate to at the place the assumption occurs, the behavior is undefined. Otherwise, the statement does nothing. In particular, the expression is not evaluated (but it is still potentially evaluated).

The purpose of an assumption is to allow compiler optimizations based on the information given.

The expression may not be a comma operator expression, but enclosing the expression in parentheses will allow the comma operator to be used.