An expression of type
cv void shall
be used only as an expression statement (
[stmt.expr]), as an operand
of a comma expression (
[expr.comma]), as a second or third operand
of
?: (
[expr.cond]), as the operand of
typeid,
noexcept, or
decltype, as
the expression in a
return statement (
[stmt.return]) for a function
with the return type
cv void, or as the operand of an explicit conversion
to type
cv void.