Better error messages for incompatibilities due to variance

[Bannerets]

Proposal

class Animal {}
class Cat extends Animal {}

class Cont<T> {}

(new Cont<Cat> : Cont<Animal>)

The above example currently displays the following error:

Cannot cast `new Cont<...>()` to `Cont` because `Cat` [1] is incompatible with `Animal` in type argument `T`

For many people, it is quite an unintuitive error message given that the Cat type itself is a subtype of Animal.

I propose error messages which show a suggestion to change class Cont<T> to class Cont<+T>. So that:

• if A <: B but C<A> </: C<B>, then an error message like "A is a subtype of B, but C is not covariant in type argument T. Did you mean to define C<+T> instead of C<T>? " should be shown.
• if B <: A but C<A> </: C<B>, then an error message like "B is a subtype of A, but C is not contravariant in type argument T. Did you mean to define C<-T> instead of C<T>?" should be shown.

(</: means "not a subtype of")

Like in Scala:

Aside of generics, it also applies to object properties:

class Animal {}
class Cat extends Animal {}
declare var obj: { a: Cat }
(obj : { a: Animal })

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Related to #3998.

[SamChou19815]

Reopen since this one is not solved. The array case is solved.