freezing indexed objects causes compile error

[kieferrm]

See microsoft/vscode#16819

TypeScript Version: typescript@2.1.4-insiders.20161206

• VSCode Version: Code - Insiders 1.8.0-insider (0d65ced0fa7398dc9d50c39a23a2a8cc8406ff18, 2016-12-07T07:05:09.124Z)
• OS Version: Darwin x64 15.6.0

Code

export interface Foo {
	property: boolean;
	[key: string]: any;
}

function freezer(bar: Foo): Foo {
	return Object.freeze(bar);
}

freezer(null);

Expected behavior:
No erros are shown in the vscode editor

Actual behavior:

The error goes away if property or the index is removed from Foo.

[mhegazy]

The issue is that keyof Foo in the exampe above is string and no"property" | string. we would like to try changing this behavior.

[kieferrm]

@mhegazy I'm not sure I understand if that implies that the code example is incorrect. If so, can you point out how the code should be changed to avoid the error?

[mhegazy]

freeze is defined to use a mapped Readonly. Mapped types generate a copy of the type using known keys (i.e. keyof Foo in this example). Since it is "property" | string and string is a super type of "property", "property" gets lost, and the new type created is missing the known property..

so long story short, this behaves as designed, but we need to revisit the design.

for now, you can cast the result to Foo, e.g.

function freezer(bar: Foo): Foo {
	return <Foo>Object.freeze(bar);
}