Typed function allows returning of objects with unknown properties

[mpk]

TypeScript Version: 2.6.2

Code

interface TestInterface {
    testValue: boolean;
}

let testFunc: () => TestInterface;

// Compiler allows returning unknown properties
testFunc = function() {
    return { testValue: true, otherValue: 7 };
}

// When the return type is explicitly specified, compiler warns about unknown properties
testFunc = function(): TestInterface {
    return { testValue: true, otherValue: 8 };
}

Playground

Expected behavior:
Since the testFunc has type annotation that specifies return value TestInterface, I expect that I will be able to return only objects with properties specified in TestInterface. However, the compiler allows me to return properties not specified in TestInterface.

To achieve my expected behavior, I need to explicitly specify the function return value. I think the compiler should be able to infer this from the type annotation.

Actual behavior:
Compiler does not complain about unknown object properties.

[j-oliveras]

This works as intended. See https://github.com/Microsoft/TypeScript/wiki/FAQ#what-is-structural-typing

[RyanCavanaugh]

Duplicate of #7547