Functions returned by Function.prototype.bind() have non-configurable lengths

[jackhorton]

Example:

Object.getOwnPropertyDescriptor(function (a, b) { return 0; }, "length").configurable // true
Object.getOwnPropertyDescriptor(function (a, b) { return 0; }.bind({}), "length").configurable // false

This is blocking tests 10.3.2_1_a_L15.js, 12.3.2_1_a_L15, 11.3.2_1_a_L15, 10.2.2_L15.js, 12.2.2_L15.js, and 11.2.2_L15.js in test262/test/intl402

/cc @bterlson

[jackhorton]

There's a whole lot of special casing in BoundFunction.cpp for not allowing things to happen to the length property (its hard-coded to be non-configurable, non-enumerable, non-writable, and non-delete-able).

[dilijev]

Removing Intl-ICU tag since this applies to non-ICU Intl as well.

[bterlson]

Possibly related: #3606.

[akroshg]

well #3606 didn't address the boundfunction scenario. But it can easily be done by returning true in the IsConfigurable function.

[jackhorton]

Removing that if block definitely resolves the test262 failures, but I didn't test how that impacted actually trying to configure it. There's enough special casing about the length property in BoundFunction.cpp that I would imagine it would still fail in relatively interesting ways.

[akroshg]

even though the length is marked as configurable, we actually don't honor it's configurability.

        function foo() { }
        delete foo.length;
        print(foo.hasOwnProperty("length"));

this should yield false, but it prints true

[rhuanjl]

Taking a look at this, there seem to be several related problems:

Issue	Normal Function	Bound Function
Length should be configurable	passes	fails
Deleting length should succeed	fails	fails
Deleting length should not throw	fails (in strict mode)	fails (in strict mode)
value after deletion = 0	fails	fails
using defineProperty to redefine length	fails	fails

These errors appear to relate to the mechanism for handling length:

• script functions do not have a length property
• instead traps for length are used within hasProperty, getProperty, delete property etc. to provide the correct answers
• the traps don't handle deletion or redefinition
• bizarrely the trap in the delete method has a throwIfStrict mode

I'll look at fixing this by making length into a normal property and getting rid of the trap(s).