Assignability check skipped for unique symbols

[rbuckton]

TypeScript Version: 2.8.0-dev.20180202

In #20898 @rozzzly discovered a bug that was tangential to that issue:

On a side note, I think I may have found a bug. So while typing up this issue, I found that I am able to achieve the desired behavior by using an intersection with a "tagged" object literal:

export type FOO = symbol & { FOO: true };
export const FOO: FOO = Symbol('FOO'); // ❓️ no error

We also don't see the error for declarations like:

const FOO: string = Symbol();

Expected behavior:

This should report the error: error TS2322: Type 'unique symbol' is not assignable to type 'symbol & { FOO: true; }'.

Actual behavior:

No error is reported. This is because getESSymbolLikeTypeForNode uses the type property on SymbolLinks which already has a type from the declaration, so it gets symbol & { FOO: true }.

Related Issues:

• Allow unique symbols to be interchangeable with types. Also assignability bug with Symbol(). #20898