Strange error implementing interface with class expression

TypeScript Version: typescript@2.7.0-dev.20171108

Code

export interface I<T> {
	one(): T | undefined;
	many(): T[];
}

function f(): { new<T>(): I<T> } {
	return class X<T> {
		one(): T { return <any>null; }
		many(): T[] { return <any>null; }
	};
}

Expected behavior:

No error.

Actual behavior:

src/a.ts(7,2): error TS2322: Type 'typeof X' is not assignable to type 'new <T>() => I<T>'.
  Type 'X<T | undefined>' is not assignable to type 'I<T>'.
    Types of property 'many' are incompatible.
      Type '() => (T | undefined)[]' is not assignable to type '() => T[]'.
        Type '(T | undefined)[]' is not assignable to type 'T[]'.
          Type 'T | undefined' is not assignable to type 'T'.
            Type 'undefined' is not assignable to type 'T'.

We get the error in many() despite one() being the method with a different return type.