=== tests/cases/compiler/bestCommonTypeReturnStatement.ts ===
interface IPromise<T> {
>IPromise : IPromise<T>
>T : T

    then(successCallback: (promiseValue: T) => any, errorCallback?: (reason: any) => any): IPromise<any>;
>then : (successCallback: (promiseValue: T) => any, errorCallback?: (reason: any) => any) => IPromise<any>
>successCallback : (promiseValue: T) => any
>promiseValue : T
>T : T
>errorCallback : (reason: any) => any
>reason : any
>IPromise : IPromise<T>
}

function f() {
>f : () => IPromise<void>

    if (true) return b();
>true : boolean
>b() : IPromise<void>
>b : () => IPromise<void>

    return d();
>d() : IPromise<any>
>d : () => IPromise<any>
}


function b(): IPromise<void> { return null; }
>b : () => IPromise<void>
>IPromise : IPromise<T>
>null : null

function d(): IPromise<any> { return null; }
>d : () => IPromise<any>
>IPromise : IPromise<T>
>null : null