Generic type extended in method not inferred without cast

[connor4312]

TypeScript Version: 3.8.0-dev.20191121

Search Terms: unknown' is not assignable to type, interface cast, unknown generic

Code

interface IFoo<T> {
    run<R extends T>(value: R): R;
}

class Foo<T> implements IFoo<T> {
    run<R extends T>(value: R): R { return value; }
}

declare function bar<T>(foo: IFoo<T>): void;

const works: IFoo<string> = new Foo<string>();
const doesNotWork = new Foo<string>();

bar(works);
bar(doesNotWork);

Expected behavior: I should expect bar to correctly infer the generic type in IFoo when given a Foo<string>

Actual behavior:

Argument of type 'Foo<string>' is not assignable to parameter of type 'IFoo<unknown>'.
  Types of property 'run' are incompatible.
    Type '<R extends string>(value: R) => R' is not assignable to type '<R extends unknown>(value: R) => R'.
      Types of parameters 'value' and 'value' are incompatible.
        Type 'R' is not assignable to type 'string'.
          Type 'unknown' is not assignable to type 'string'.

Playground Link: Playground Link

Related Issues:

[RyanCavanaugh]

Generic constraints aren't inference sites (and really shouldn't be), so this is effectively https://github.com/microsoft/TypeScript/wiki/FAQ#why-doesnt-type-inference-work-on-this-interface-interface-foot--

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.