TypeScript narrowed union is lost inside function scopes

env.ts

export type Foo = {
    foo: string;
};
export type Bar = {
    bar: string;
};

export type Value = Foo | Bar;

export declare const value: Value;

export declare const checkIsFoo: (value: Value) => value is Foo;

// Test (internal)

{
    const fn = (value: Value) => {
        if (checkIsFoo(value)) {
            const test1: Foo = value;
            const fn = () => {
                // No error, good
                const test2: Foo = value;
            };
        }
    };
}

{
    if (checkIsFoo(value)) {
        const test1: Foo = value;
        const fn = () => {
            // No error, good
            const test2: Foo = value;
        };
    }
}

test.ts

// Explanation:
// https://github.com/microsoft/TypeScript/pull/8849
// https://github.com/Microsoft/TypeScript/issues/9258

import { checkIsFoo, Foo, Value, value } from './env';

// Test (external)

{
    const fn = (value: Value) => {
        if (checkIsFoo(value)) {
            const test1: Foo = value;
            const fn = () => {
                // No error, good
                const test2: Foo = value;
            };
        }
    };
}

{
    if (checkIsFoo(value)) {
        const test1: Foo = value;
        const fn = () => {
            // Unexpected error, bad
            const test2: Foo = value;
        };
    }
}

// Workaround

{
    if (checkIsFoo(value)) {
        const foo = value;
        const test1: Foo = foo;
        const fn = () => {
            // No error, good
            const test2: Foo = foo;
        };
    }
}