No type inference from callback in mapped type

TypeScript Version: 2.7.0-dev.20180118

Code

declare function conforms<T>(source: { [K in keyof T]: (val: T[K]) => boolean }): (value: T) => boolean;
conforms({ foo: (v: string) => false })({ foo: "foo" });

Expected behavior:

No error. T is inferred as { foo: string }.

Actual behavior:

T is inferred as { foo: {} }.

src/a.ts(2,10): error TS2345: Argument of type '{ foo: (v: string) => boolean; }' is not assignable to parameter of type '{ foo: (val: {}) => boolean; }'.
  Types of property 'foo' are incompatible.
    Type '(v: string) => boolean' is not assignable to type '(val: {}) => boolean'.
      Types of parameters 'v' and 'val' are incompatible.
        Type '{}' is not assignable to type 'string'.

Worked in 2.7.0-dev.20180116.

Detected in lodash on DefinitelyTyped.

@ahejlsberg It looks like this is a change from #21157. If I run git revert 1ae0b461f88e9a68c2b2ad51519611a2cc292fec (the first commit in that PR) I can compile without errors when testing with a local build.