Invalid emit for class expression with computed property name in parameter initializer

[ajafff]

TypeScript Version: 3.2.1

Search Terms:

Code

// @target: ES6
function test(
    x = class {
        [y] = 1
    },
    y: 'foo',
) {}

Expected behavior:

Calling test(undefined, "foo") executes without an error.

Actual behavior:

Runtime error ReferenceError: _b is not defined.

The helper variables _a and _b are declared inside the function body, but parameter initializers cannot reference declarations in the function body. I guess this could be fixed by wrapping the emit of the class expression in an IIFE.

"use strict";
function test(x = (_b = class {
        constructor() {
            this[_a] = 1;
        }
    },
    _a = y,
    _b), y) { var _a, _b; }

Note: The same code downleveled to ES5 works as expected as the parameter initializer is emitted inside the function body.

Playground Link: https://agentcooper.github.io/typescript-play/?target=2#code/GYVwdgxgLglg9mABFApgZygCgFCL4gD0QF5EIAbAQzTUQG9d8mBtATwF0TEBGRvAXwA0fRKwBciAOTA4cScICU9ftiA

Related Issues:

[rbuckton]

The example is invalid in ES2015 and up as each parameter initializer essentially gets its own lexical scope and cannot reference other parameters or variables in the body. At some point we added an error to this effect: