Class invariants without parentheses are not supported

[linkrope]

See http://dlang.org/spec/class.html#invariants

The alternative used in the Language Reference

invariant
{
    assert(1 <= day && day <= 31);
    assert(0 <= hour && hour < 24);
}

is not supported:

Syntax error on token "invariant", expected OPEN_PARENS after.