Note [Polymorphisation of linear fields]

The choice in Note [Data constructors are linear by default] has an impact on
backwards compatibility. Consider

    map Just

We have

    map :: (a -> b) -> f a -> f b
    Just :: a %1 -> Just a

Types don't match, we should get a type error. But this is legal Haskell 98
code! Bad! Bad! Bad!

It could be solved with subtyping, but subtyping doesn't combine well with
polymorphism. Instead, we generalise the type of Just, when used as term:

   Just :: forall {p}. a %p-> Just a

This is solely a concern for higher-order code like this: when called fully
applied linear constructors are more general than constructors with unrestricted
fields. In particular, linear constructors can always be eta-expanded to their
Haskell 98 type. This is explained in the paper (but there, we had a different
strategy to resolve this type mismatch in higher-order code. It turned out to be
insufficient, which is explained in the wiki page as well as the proposal).

We only generalise linear fields this way: fields with multiplicity Many, or
other multiplicity expressions are exclusive to -XLinearTypes, hence don't have
backward compatibility implications.

The implementation is described in Note [Typechecking data constructors]
in GHC.Tc.Gen.Head.

More details in the proposal.