tests/cases/compiler/optionalFunctionArgAssignability.ts(7,1): error TS2322: Type '<U>(onFulFill?: (value: number) => U, onReject?: (reason: any) => U) => Promise<U>' is not assignable to type '<U>(onFulfill?: (value: string) => U, onReject?: (reason: any) => U) => Promise<U>'.
  Types of parameters 'onFulFill' and 'onFulfill' are incompatible.
    Type '(value: number) => any' is not assignable to type '(value: string) => any'.
      Types of parameters 'value' and 'value' are incompatible.
        Type 'number' is not assignable to type 'string'.


==== tests/cases/compiler/optionalFunctionArgAssignability.ts (1 errors) ====
    interface Promise<T> {
        then<U>(onFulfill?: (value: T) => U, onReject?: (reason: any) => U): Promise<U>;
    }
     
    var a = function then<U>(onFulfill?: (value: string) => U, onReject?: (reason: any) => U): Promise<U> { return null };
    var b = function then<U>(onFulFill?: (value: number) => U, onReject?: (reason: any) => U): Promise<U> { return null };
    a = b; // error because number is not assignable to string
    ~
!!! error TS2322: Type '<U>(onFulFill?: (value: number) => U, onReject?: (reason: any) => U) => Promise<U>' is not assignable to type '<U>(onFulfill?: (value: string) => U, onReject?: (reason: any) => U) => Promise<U>'.
!!! error TS2322:   Types of parameters 'onFulFill' and 'onFulfill' are incompatible.
!!! error TS2322:     Type '(value: number) => any' is not assignable to type '(value: string) => any'.
!!! error TS2322:       Types of parameters 'value' and 'value' are incompatible.
!!! error TS2322:         Type 'number' is not assignable to type 'string'.