Wrong output for arrow function returning class that extends argument

[chge]

The following code:

const f = (b) => class a extends b {};

Produces the following incorrect output:

var f = function (b) (function (b) {
// ... the rest is correct

Possible workaround is to use parentheses:

const f = (b) => (class a extends b {});

...or to to use ordinary function:

const f = function(b) { return class a extends b {}; };

Caught this while trying to transpile asn.js with such syntax.