Typechecking Partial<> works differently for generic types

[jauco]

TypeScript Version: 2.1.1 (what's running on the playground)

Code
This works

interface Data {
    a?: "a"|"b",
    b: number,
    c: "a" | "b"
}

type Patch = Partial<Data> 
function copyFields(target: Data, source: Patch): Data {
    for (let id in source) {
        target[id] = source[id];
    }
    return target;
}

function makesrc(): Data { return {b: 1, c: "a"}}


/*1*/copyFields(makesrc(), {d: "d"}); //Should (and does) give an error
/*2*/copyFields(makesrc(), {a: "d"}); //Should (and does) give an error
/*3*/copyFields(makesrc(), {c: "d"}); //Should (and does) give an error

//I don't want to specify all the source properties 
/*4*/copyFields(makesrc(), {b: 2}); //Should not (and doesn't) give an error
/*5*/copyFields(makesrc(), {a: "b"}); //Should not (and doesn't) give an error

But the generic version doesn't

interface Data {
    a?: "a"|"b",
    b: number,
    c: "a" | "b"
}

function copyFields<T>(target: T, source: Partial<T>): T {
    for (let id in source) {
        target[id] = source[id];
    }
    return target;
}

function makesrc(): Data { return {b: 1, c: "a"}}


/*1*/copyFields(makesrc(), {d: "d"}); //Should (and does) give an error
/*2*/copyFields(makesrc(), {a: "d"}); //Should (and does) give an error
/*3*/copyFields(makesrc(), {c: "d"}); //Should (BUT DOES NOT) give an error

//I don't want to specify all the source properties 
/*4*/copyFields(makesrc(), {b: 2}); //Should not (and doesn't) give an error
/*5*/copyFields(makesrc(), {a: "b"}); //Should not (BUT DOES) give an error

I expected no change in type safety in the generic version.

[aluanhaddad]

Error in nightly.

[timocov]

Workaround. Specify type T:

/*3*/copyFields<Data>(makesrc(), {c: "d"});
/*5*/copyFields<Data>(makesrc(), {a: "b"});

[mhegazy]

duplicate of #12633, fixed by #12640. should be working as intended in TS 2.1.4

[jauco]

Thanks! Sorry for reporting the duplicate. I found it hard to find the right keywords to search for.