String Literal Type does not work

[sevenryze]

TypeScript Version: 2.7.0-dev.201xxxxx

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
interface TypeA {
    test: "test1" | "test2" | "test3"
}

function fn(param: TypeA) {

}

// Check pass
fn({
    test: "test1"
});

// The same object, but passed by object reference
let obj = {
    test: "test1"
};

/**
 * Error:(26, 4) TS2345: Argument of type '{ test: string; }' is not assignable to parameter of type 'TypeA'.
 *   Types of property 'test' are incompatible.
 *   Type 'string' is not assignable to type '"test1" | "test2" | "test3"'.
 */
fn(obj);

Expected behavior:
Check pass
Actual behavior:
I get a TS2345

[ahejlsberg]

This is working as intended. The argument in the call to fn is contextually typed by TypeA and therefore it is known that the test property has a literal type. However, in the let assignment there is no contextual type, so the type of the test property is widened to string because the property is a mutable location.

It works if you add a type annotation (thus providing a contextual type):

let obj: TypeA = {
    test: "test1"
};

Duplicate of #17363 and others. For a discussion of the widening rules see #10676.

[sevenryze]

Thanks for your promoting reply.