tests/cases/conformance/parser/ecmascript5/IndexSignatures/parserIndexSignature8.ts(1,13): error TS1023: An index signature parameter type must be 'string' or 'number'. tests/cases/conformance/parser/ecmascript5/IndexSignatures/parserIndexSignature8.ts(2,14): error TS1023: An index signature parameter type must be 'string' or 'number'. ==== tests/cases/conformance/parser/ecmascript5/IndexSignatures/parserIndexSignature8.ts (2 errors) ==== var foo: { [index: any]; }; // expect an error here ~~~~~ !!! error TS1023: An index signature parameter type must be 'string' or 'number'. var foo2: { [index: RegExp]; }; // expect an error here ~~~~~ !!! error TS1023: An index signature parameter type must be 'string' or 'number'.