tests/cases/conformance/es6/Symbols/symbolProperty17.ts(3,6): error TS1023: An index signature parameter type must be 'string' or 'number'. ==== tests/cases/conformance/es6/Symbols/symbolProperty17.ts (1 errors) ==== interface I { [Symbol.iterator]: number; [s: symbol]: string; ~ !!! error TS1023: An index signature parameter type must be 'string' or 'number'. "__@iterator": string; } var i: I; var it = i[Symbol.iterator];