'symbol' is not allowed as an index signature parameter type #7667

TypeScript Version:

1.8.7 (ES6 mode)

Code

type SymbolKeysGiveNumbers = { [x: symbol]: number };

Expected behavior:
Should work

Actual behavior:
Error: an index signature parameter must be 'string' or 'number'

However:
Code

let x: { [key: string]: any };
x[Symbol("abcd")] = 12;

Expected behavior:
Should not work based on the error above

Actual behavior:
Works

Conclusion
Symbol type should be allowed as an index signature parameter type as in practice it is accepted as a valid index key type. Even the error message when doing something like:

let x: { [key: string]: any };
x[true] = 1;

includes 'symbol' as a valid option:

An index expression argument must be of type 'string', 'number', 'symbol' or 'any'