No error for assignment to undefined inside extends expression

[sunlili]

Hello,
The following code behaves strangely (inconsistent with other engines).

class cc extends ( x = function () {
    console.log("Hi");
}) {
};

In Edge, cc is defined normally and x is defined too.
However, in V8 and Firefox, a ReferenceError is reported:
ReferenceError: assignment to undeclared variable x

BT group
2018.6.26

[zenparsing]

Chakra appears to correctly execute the extends expression in strict mode:

> eshost -e "class cc extends (delete x) {}"
#### ch
SyntaxError: Calling delete on expression not allowed in strict mode

But is not correctly throwing on assignment to an undeclared variable in the extends expression:

class cc extends (x = class {}) {} // Should throw a ReferenceError