tests/cases/conformance/es6/Symbols/symbolProperty24.ts(5,7): error TS2420: Class 'C' incorrectly implements interface 'I'. Types of property '[Symbol.toPrimitive]' are incompatible. Type '() => string' is not assignable to type '() => boolean'. Type 'string' is not assignable to type 'boolean'. ==== tests/cases/conformance/es6/Symbols/symbolProperty24.ts (1 errors) ==== interface I { [Symbol.toPrimitive]: () => boolean; } class C implements I { ~ !!! error TS2420: Class 'C' incorrectly implements interface 'I'. !!! error TS2420: Types of property '[Symbol.toPrimitive]' are incompatible. !!! error TS2420: Type '() => string' is not assignable to type '() => boolean'. !!! error TS2420: Type 'string' is not assignable to type 'boolean'. [Symbol.toPrimitive]() { return ""; } }