tests/cases/compiler/optionalArgsWithDefaultValues.ts(1,25): error TS1015: Parameter cannot have question mark and initializer. tests/cases/compiler/optionalArgsWithDefaultValues.ts(4,27): error TS1015: Parameter cannot have question mark and initializer. tests/cases/compiler/optionalArgsWithDefaultValues.ts(5,28): error TS1015: Parameter cannot have question mark and initializer. tests/cases/compiler/optionalArgsWithDefaultValues.ts(8,10): error TS1015: Parameter cannot have question mark and initializer. tests/cases/compiler/optionalArgsWithDefaultValues.ts(9,13): error TS1015: Parameter cannot have question mark and initializer. ==== tests/cases/compiler/optionalArgsWithDefaultValues.ts (5 errors) ==== function foo(x: number, y?:boolean=false, z?=0) {} ~ !!! error TS1015: Parameter cannot have question mark and initializer. class CCC { public foo(x: number, y?:boolean=false, z?=0) {} ~ !!! error TS1015: Parameter cannot have question mark and initializer. static foo2(x: number, y?:boolean=false, z?=0) {} ~ !!! error TS1015: Parameter cannot have question mark and initializer. } var a = (x?=0) => { return 1; }; ~ !!! error TS1015: Parameter cannot have question mark and initializer. var b = (x, y?:number = 2) => { x; }; ~ !!! error TS1015: Parameter cannot have question mark and initializer.