In #41995 the type of `TrackByFunction` was changed such that the
declaration of a `trackBy` function did not cause the item type to be
widened to the `trackBy`'s item type, which may be a supertype of the
iterated type. This has introduced situations where the template type
checker is now reporting errors for cases where a `trackBy` function is
no longer assignable to `TrackByFunction`.
This commit fixes the error by also including the item type `T` in
addition to the constrained type parameter `U`, allowing TypeScript to
infer an appropriate `T`.
Fixes#42609
PR Close#42692
51156f3f07