Function param with generic type are not recognized correctly

[k8w]

TypeScript Version: 2.7.0-dev.201xxxxx
2.8.3

Search Terms:

Code

class Protocol<T>{}

interface ReqTest {
    abcde: string;
}

const PtlTest = new Protocol<ReqTest>();

function request<T>(ptl: Protocol<T>, req: T) { }

request(PtlTest, {})

Expected behavior:
Compile error.
Because request(PtlTest, {}), param 2 {} not match ReqTest

Actual behavior:
Compile success

Playground Link:

Related Issues:

You need an explicit type argument here, because {} is a valid inference. PtlTest validly a Protocol<{}> (T is never even used, so everything but null or undefined is aProtocol<T> for any T), and {} of course is a valid {}.

[k8w]

@Andy-MS shouldn't T be infered from PtlDemo?
Actually it works well in previous TypeScript version.

[k8w]

@Andy-MS shouldn't T be infered from PtlDemo?
Actually it works well in previous TypeScript version.

I don't see a compile error in any of typescript@2.6, typescript@2.7, typescript@next. Protocol is an empty type, so almost anything is assignable to it.

[k8w]

function test<T>(value1: T, value2: T) {
    return value2
}
test({ a: 1 }, {});

@Andy-MS Just see this.
It would compile error at typescript@2.6:

error TS2345: Argument of type '{}' is not assignable to parameter of type '{ a: number; }'.
Property 'a' is missing in type '{}'.
But compile succ at typescript@2.8

And it's strange that it works well if it is this:

function test<T>(value1: T, value2: T) {
    return value2
}
test(<{ a: number }>{ a: 1 }, {});

[k8w]

class Protocol<T>{ }

interface ReqTest {
    abcde: string;
}

const PtlTest: Protocol<ReqTest> = new Protocol<ReqTest>();

function request<T>(ptl: Protocol<T>, req: T): T { return req; }

request(PtlTest, {})

@Andy-MS And even if I give PtlTest a explicit type Protocol<ReqTest> it still not works.
(and T is used for return type)
{} can not ensure that T are inferenced as the same.

I'll tentatively mark this as a bug due to the inconsistency in:

declare function equal<T>(a: T, b: T): boolean;
equal("", 0); // Error
equal({ a: 1 }, { b: 2 }); // No error

[mhegazy]

See more about the rationals here in https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#improved-type-inference-for-object-literals

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.