input.tsx

// valid JSX
<in T>() => {}</in>;

type Covariant<out T> = {
    x: T;
}

declare let super_covariant: Covariant<unknown>;
declare let sub_covariant: Covariant<string>;

super_covariant = sub_covariant;
sub_covariant = super_covariant;  // Error

type Contravariant<in T> = {
    f: (x: T) => void;
}

declare let super_contravariant: Contravariant<unknown>;
declare let sub_contravariant: Contravariant<string>;

super_contravariant = sub_contravariant;  // Error
sub_contravariant = super_contravariant;

type Invariant<in out T> = {
    f: (x: T) => T;
}

declare let super_invariant: Invariant<unknown>;
declare let sub_invariant: Invariant<string>;

super_invariant = sub_invariant;  // Error
sub_invariant = super_invariant;  // Error

// Variance of various type constructors

type T10<out T> = T;
type T11<in T> = keyof T;
type T12<out T, out K extends keyof T> = T[K];
type T13<in out T> = T[keyof T];

// Variance annotation errors

type Covariant1<in T> = {  // Error
    x: T;
}

type Contravariant1<out T> = keyof T;  // Error

type Contravariant2<out T> = {  // Error
    f: (x: T) => void;
}

type Invariant1<in T> = {  // Error
    f: (x: T) => T;
}

type Invariant2<out T> = {  // Error
    f: (x: T) => T;
}

// Variance in circular types

type Foo1<in T> = {  // Error
    x: T;
    f: FooFn1<T>;
}

type FooFn1<T> = (foo: Bar1<T[]>) => void;

type Bar1<T> = {
    value: Foo1<T[]>;
}

type Foo2<out T> = {  // Error
    x: T;
    f: FooFn2<T>;
}

type FooFn2<T> = (foo: Bar2<T[]>) => void;

type Bar2<T> = {
    value: Foo2<T[]>;
}

type Foo3<in out T> = {
    x: T;
    f: FooFn3<T>;
}

type FooFn3<T> = (foo: Bar3<T[]>) => void;

type Bar3<T> = {
    value: Foo3<T[]>;
}

// Wrong modifier usage

type T20<public T> = T;  // Error
type T21<in out in T> = T;  // Error
type T22<in out out T> = T;  // Error
type T23<out in T> = T;  // Error

declare function f1<in T>(x: T): void;  // Error
declare function f2<out T>(): T;  // Error

class C {
    in a = 0;  // Error
    out b = 0;  // Error
}

// Interface merging

interface Baz<out T> {}
interface Baz<in T> {}

declare let baz1: Baz<unknown>;
declare let baz2: Baz<string>;

baz1 = baz2;  // Error
baz2 = baz1;  // Error

// Repro from #44572

interface Parent<out A> {
    child: Child<A> | null;
    parent: Parent<A> | null;
}

interface Child<A, B = unknown> extends Parent<A> {
    readonly a: A;
    readonly b: B;
}

function fn<A>(inp: Child<A>) {
    const a: Child<unknown> = inp;
}

const pu: Parent<unknown> = { child: { a: 0, b: 0, child: null, parent: null }, parent: null };
const notString: Parent<string> = pu;  // Error

// Repro from comment in #44572

declare class StateNode<TContext, in out TEvent extends { type: string }> {
    _storedEvent: TEvent;
    _action: ActionObject<TEvent>;
    _state: StateNode<TContext, any>;
}

interface ActionObject<TEvent extends { type: string }> {
    exec: (meta: StateNode<any, TEvent>) => void;
}

declare function createMachine<TEvent extends { type: string }>(action: ActionObject<TEvent>): StateNode<any, any>;

declare function interpret<TContext>(machine: StateNode<TContext, any>): void;

const machine = createMachine({} as any);

interpret(machine);

declare const qq: ActionObject<{ type: "PLAY"; value: number }>;

createMachine<{ type: "PLAY"; value: number } | { type: "RESET" }>(qq);  // Error