covariantCallbacks.ts(12,5): error TS2322: Type 'P<A>' is not assignable to type 'P<B>'.
  Property 'b' is missing in type 'A' but required in type 'B'.
covariantCallbacks.ts(17,5): error TS2322: Type 'Promise<A>' is not assignable to type 'Promise<B>'.
  Property 'b' is missing in type 'A' but required in type 'B'.
covariantCallbacks.ts(30,5): error TS2322: Type 'AList1' is not assignable to type 'BList1'.
  Types of property 'forEach' are incompatible.
    Type '(cb: (item: A) => void) => void' is not assignable to type '(cb: (item: B) => void) => void'.
      Types of parameters 'cb' and 'cb' are incompatible.
        Types of parameters 'item' and 'item' are incompatible.
          Property 'b' is missing in type 'A' but required in type 'B'.
covariantCallbacks.ts(43,5): error TS2322: Type 'AList2' is not assignable to type 'BList2'.
  Types of property 'forEach' are incompatible.
    Types of parameters 'cb' and 'cb' are incompatible.
      Type 'void' is not assignable to type 'boolean'.
covariantCallbacks.ts(56,5): error TS2322: Type 'AList3' is not assignable to type 'BList3'.
  Types of property 'forEach' are incompatible.
    Type '(cb: (item: A) => void) => void' is not assignable to type '(cb: (item: A, context: any) => void) => void'.
      Types of parameters 'cb' and 'cb' are incompatible.
        Target signature provides too few arguments. Expected 2 or more, but got 1.
covariantCallbacks.ts(69,5): error TS2322: Type 'AList4' is not assignable to type 'BList4'.
  Types of property 'forEach' are incompatible.
    Type '(cb: (item: A) => A) => void' is not assignable to type '(cb: (item: B) => B) => void'.
      Types of parameters 'cb' and 'cb' are incompatible.
        Types of parameters 'item' and 'item' are incompatible.
          Property 'b' is missing in type 'A' but required in type 'B'.
covariantCallbacks.ts(98,1): error TS2322: Type 'SetLike1<(x: string) => void>' is not assignable to type 'SetLike1<(x: unknown) => void>'.
  Type '(x: string) => void' is not assignable to type '(x: unknown) => void'.
    Types of parameters 'x' and 'x' are incompatible.
      Type 'unknown' is not assignable to type 'string'.
covariantCallbacks.ts(106,1): error TS2322: Type 'SetLike2<(x: string) => void>' is not assignable to type 'SetLike1<(x: unknown) => void>'.
  The types returned by 'get()' are incompatible between these types.
    Type '(x: string) => void' is not assignable to type '(x: unknown) => void'.
      Types of parameters 'x' and 'x' are incompatible.
        Type 'unknown' is not assignable to type 'string'.


==== covariantCallbacks.ts (8 errors) ====
    // Test that callback parameters are related covariantly
    
    interface P<T> {
        then(cb: (value: T) => void): void;
    };
    
    interface A { a: string }
    interface B extends A { b: string }
    
    function f1(a: P<A>, b: P<B>) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'P<A>' is not assignable to type 'P<B>'.
!!! error TS2322:   Property 'b' is missing in type 'A' but required in type 'B'.
!!! related TS2728 covariantCallbacks.ts:8:25: 'b' is declared here.
    }
    
    function f2(a: Promise<A>, b: Promise<B>) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'Promise<A>' is not assignable to type 'Promise<B>'.
!!! error TS2322:   Property 'b' is missing in type 'A' but required in type 'B'.
!!! related TS2728 covariantCallbacks.ts:8:25: 'b' is declared here.
    }
    
    interface AList1 {
        forEach(cb: (item: A) => void): void;
    }
    
    interface BList1 {
        forEach(cb: (item: B) => void): void;
    }
    
    function f11(a: AList1, b: BList1) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'AList1' is not assignable to type 'BList1'.
!!! error TS2322:   Types of property 'forEach' are incompatible.
!!! error TS2322:     Type '(cb: (item: A) => void) => void' is not assignable to type '(cb: (item: B) => void) => void'.
!!! error TS2322:       Types of parameters 'cb' and 'cb' are incompatible.
!!! error TS2322:         Types of parameters 'item' and 'item' are incompatible.
!!! error TS2322:           Property 'b' is missing in type 'A' but required in type 'B'.
!!! related TS2728 covariantCallbacks.ts:8:25: 'b' is declared here.
    }
    
    interface AList2 {
        forEach(cb: (item: A) => boolean): void;
    }
    
    interface BList2 {
        forEach(cb: (item: A) => void): void;
    }
    
    function f12(a: AList2, b: BList2) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'AList2' is not assignable to type 'BList2'.
!!! error TS2322:   Types of property 'forEach' are incompatible.
!!! error TS2322:     Types of parameters 'cb' and 'cb' are incompatible.
!!! error TS2322:       Type 'void' is not assignable to type 'boolean'.
    }
    
    interface AList3 {
        forEach(cb: (item: A) => void): void;
    }
    
    interface BList3 {
        forEach(cb: (item: A, context: any) => void): void;
    }
    
    function f13(a: AList3, b: BList3) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'AList3' is not assignable to type 'BList3'.
!!! error TS2322:   Types of property 'forEach' are incompatible.
!!! error TS2322:     Type '(cb: (item: A) => void) => void' is not assignable to type '(cb: (item: A, context: any) => void) => void'.
!!! error TS2322:       Types of parameters 'cb' and 'cb' are incompatible.
!!! error TS2322:         Target signature provides too few arguments. Expected 2 or more, but got 1.
    }
    
    interface AList4 {
        forEach(cb: (item: A) => A): void;
    }
    
    interface BList4 {
        forEach(cb: (item: B) => B): void;
    }
    
    function f14(a: AList4, b: BList4) {
        a = b;
        b = a;  // Error
        ~
!!! error TS2322: Type 'AList4' is not assignable to type 'BList4'.
!!! error TS2322:   Types of property 'forEach' are incompatible.
!!! error TS2322:     Type '(cb: (item: A) => A) => void' is not assignable to type '(cb: (item: B) => B) => void'.
!!! error TS2322:       Types of parameters 'cb' and 'cb' are incompatible.
!!! error TS2322:         Types of parameters 'item' and 'item' are incompatible.
!!! error TS2322:           Property 'b' is missing in type 'A' but required in type 'B'.
!!! related TS2728 covariantCallbacks.ts:8:25: 'b' is declared here.
    }
    
    // Repro from #51620
    
    type Bivar<T> = { set(value: T): void }
    
    declare let bu: Bivar<unknown>;
    declare let bs: Bivar<string>;
    bu = bs;
    bs = bu;
    
    declare let bfu: Bivar<(x: unknown) => void>;
    declare let bfs: Bivar<(x: string) => void>;
    bfu = bfs;
    bfs = bfu;
    
    type Bivar1<T> = { set(value: T): void }
    type Bivar2<T> = { set(value: T): void }
    
    declare let b1fu: Bivar1<(x: unknown) => void>;
    declare let b2fs: Bivar2<(x: string) => void>;
    b1fu = b2fs;
    b2fs = b1fu;
    
    type SetLike<T> = { set(value: T): void, get(): T }
    
    declare let sx: SetLike1<(x: unknown) => void>;
    declare let sy: SetLike1<(x: string) => void>;
    sx = sy;  // Error
    ~~
!!! error TS2322: Type 'SetLike1<(x: string) => void>' is not assignable to type 'SetLike1<(x: unknown) => void>'.
!!! error TS2322:   Type '(x: string) => void' is not assignable to type '(x: unknown) => void'.
!!! error TS2322:     Types of parameters 'x' and 'x' are incompatible.
!!! error TS2322:       Type 'unknown' is not assignable to type 'string'.
    sy = sx;
    
    type SetLike1<T> = { set(value: T): void, get(): T }
    type SetLike2<T> = { set(value: T): void, get(): T }
    
    declare let s1: SetLike1<(x: unknown) => void>;
    declare let s2: SetLike2<(x: string) => void>;
    s1 = s2;  // Error
    ~~
!!! error TS2322: Type 'SetLike2<(x: string) => void>' is not assignable to type 'SetLike1<(x: unknown) => void>'.
!!! error TS2322:   The types returned by 'get()' are incompatible between these types.
!!! error TS2322:     Type '(x: string) => void' is not assignable to type '(x: unknown) => void'.
!!! error TS2322:       Types of parameters 'x' and 'x' are incompatible.
!!! error TS2322:         Type 'unknown' is not assignable to type 'string'.
    s2 = s1;
    