operator "??" does not support throw expression - by design?

[ghost]

Expression x ?? expr, intuitively, should be equivalent to x != null ? x : expr, but it is not.

x != null ? x : throw "error"; // OK
x ?? throw "error"; // Error

Is it so by design?

[JxJPu9bD]

use '?:' instead?

[eernstg]

That's a syntax error; just use x ?? (throw "error").

The underlying reason is that ?? requires an operand which is a <logicalOrExpression>, but <throwExpression> is derived directly from <expression> (so it's "more general" than <logicalOrExpression>).

[lrhn]

The precedence of throw is such that this does not work. See also #24891.

The throw operator binds weakly. It means that throw foo + bar means throw (foo + bar) and not (throw foo) + bar, because the latter is meaningless anyway.
(In hindsight, that might not have been the best choice, because it prevents things like foo ?? throw bar, and you would not write throw foo + bar anyway, and it's confusing that it works differently from await).

As Erik has said, you have to wrap your throw in parentheses.