==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignaturesWithOptionalParameters.ts (7 errors) ====
    // call signatures in derived types must have the same or fewer optional parameters as the base type
    
    interface Base { 
        a: () => number;
        a2: (x?: number) => number;
        a3: (x: number) => number;
        a4: (x: number, y?: number) => number;
        a5: (x?: number, y?: number) => number;
        a6: (x: number, y: number) => number;
    }
    var b: Base;
    
    var a: () => number;
        a = () => 1 // ok, same number of required params
        a = (x?: number) => 1; // ok, same number of required params
        a = (x: number) => 1; // error, too many required params
        ~
!!! Type '(x: number) => number' is not assignable to type '() => number'.
        a = b.a; // ok
        a = b.a2; // ok
        a = b.a3; // error
        ~
!!! Type '(x: number) => number' is not assignable to type '() => number'.
        a = b.a4; // error
        ~
!!! Type '(x: number, y?: number) => number' is not assignable to type '() => number'.
        a = b.a5; // ok
        a = b.a6; // error
        ~
!!! Type '(x: number, y: number) => number' is not assignable to type '() => number'.
    
    var a2: (x?: number) => number; 
        a2 = () => 1; // ok, same number of required params
        a2 = (x?: number) => 1; // ok, same number of required params
        a2 = (x: number) => 1; // ok, same number of params
        a2 = b.a; // ok
        a2 = b.a2; // ok
        a2 = b.a3; // ok, same number of params
        a2 = b.a4; // ok, excess params are optional in b.a3
        a2 = b.a5; // ok
        a2 = b.a6; // error
        ~~
!!! Type '(x: number, y: number) => number' is not assignable to type '(x?: number) => number'.
    
    var a3: (x: number) => number; 
        a3 = () => 1; // ok, fewer required params
        a3 = (x?: number) => 1; // ok, fewer required params
        a3 = (x: number) => 1; // ok, same number of required params
        a3 = (x: number, y: number) => 1;  // error, too many required params
        ~~
!!! Type '(x: number, y: number) => number' is not assignable to type '(x: number) => number'.
        a3 = b.a; // ok
        a3 = b.a2; // ok
        a3 = b.a3; // ok
        a3 = b.a4; // ok
        a3 = b.a5; // ok
        a3 = b.a6; // error
        ~~
!!! Type '(x: number, y: number) => number' is not assignable to type '(x: number) => number'.
    
    var a4: (x: number, y?: number) => number;
        a4 = () => 1; // ok, fewer required params
        a4 = (x?: number, y?: number) => 1; // ok, fewer required params
        a4 = (x: number) => 1; // ok, same number of required params
        a4 = (x: number, y: number) => 1; // ok, same number of params
        a4 = b.a; // ok
        a4 = b.a2; // ok
        a4 = b.a3; // ok
        a4 = b.a4; // ok
        a4 = b.a5; // ok
        a4 = b.a6; // ok, same number of params
    
    var a5: (x?: number, y?: number) => number;
        a5 = () => 1; // ok, fewer required params
        a5 = (x?: number, y?: number) => 1; // ok, fewer required params
        a5 = (x: number) => 1; // ok, fewer params in lambda
        a5 = (x: number, y: number) => 1;  // ok, same number of params
        a5 = b.a; // ok
        a5 = b.a2; // ok
        a5 = b.a3; // ok, fewer params in b.a3
        a5 = b.a4; // ok, same number of params
        a5 = b.a5; // ok
        a5 = b.a6; // ok, same number of params