tests/cases/conformance/es6/Symbols/symbolProperty32.ts(7,6): error TS1023: An index signature parameter type must be 'string' or 'number'. ==== tests/cases/conformance/es6/Symbols/symbolProperty32.ts (1 errors) ==== class C1 { [Symbol.toStringTag]() { return { x: "" }; } } class C2 extends C1 { [s: symbol]: () => { x: number }; ~ !!! error TS1023: An index signature parameter type must be 'string' or 'number'. }