Missing an early error of ES2016 (ES7)

[mysticatea]

function foo(a = 1) {
  "use strict"
}

In ES2016, this code raises a syntax error.

It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of FormalParameters is false.
https://tc39.github.io/ecma262/2016/#sec-function-definitions-static-semantics-early-errors

But Acorn (current HEAD: 7323217) succeeded parsing the code.

(Note: this is a valid syntax in ES2015.)

---

"use strict";

var acorn = require("./dist/acorn");
var ast = acorn.parse(`
function foo(a = 1) {
  "use strict"
}
`, {ecmaVersion: 7, sourceType: "script"});

console.log(JSON.stringify(ast, null, 4));

> node test.js
{
    "type": "Program",
    "start": 0,
    "end": 40,
    "body": [
        {
            "type": "FunctionDeclaration",
            "start": 1,
            "end": 39,
            "id": {
                "type": "Identifier",
                "start": 10,
                "end": 13,
                "name": "foo"
            },
            "generator": false,
            "expression": false,
            "params": [
                {
                    "type": "AssignmentPattern",
                    "start": 14,
                    "end": 19,
                    "left": {
                        "type": "Identifier",
                        "start": 14,
                        "end": 15,
                        "name": "a"
                    },
                    "right": {
                        "type": "Literal",
                        "start": 18,
                        "end": 19,
                        "value": 1,
                        "raw": "1"
                    }
                }
            ],
            "body": {
                "type": "BlockStatement",
                "start": 21,
                "end": 39,
                "body": [
                    {
                        "type": "ExpressionStatement",
                        "start": 25,
                        "end": 37,
                        "expression": {
                            "type": "Literal",
                            "start": 25,
                            "end": 37,
                            "value": "use strict",
                            "raw": "\"use strict\""
                        }
                    }
                ]
            }
        }
    ],
    "sourceType": "script"
}

[marijnh]

We're not supporting much of ES2016 yet.

Do you happen to know why this innocent-looking form is marked as an early error?

[mysticatea]

Do you happen to know why this innocent-looking form is marked as an early error?

From a TC39 Meeting Note: https://github.com/rwaldron/tc39-notes/blob/master/es7/2015-07/july-29.md#611-the-scope-of-use-strict-with-respect-to-destructuring-in-parameter-lists

The main reason seems performance.

[marijnh]

Thanks for the link!

[nzakas]

@marijnh ES2016 is ES7, which Acorn supports (I helped add it,ecmaVersion: 7, it was just the ** operator).

A bit more info from my research: the problem is that strict mode is supposed to apply to function parameters when "use strict" is inside a function. That was okay in ES5 because the only real strict mode check affecting parameters was the double parameter error. However, in ES6, you can have default parameter values which might be functions or use values such as this. So the engine would have to parse through functions with potential nesting to n levels depth only to get to the end and find a "use strict" in the function body and have to rewind and start over again.

[nzakas]

I'll try to add this.

[RReverser]

@nzakas Thanks for the thorough explanation!