Sebastian Graf authored Nov 23, 2020 and Marge Bot committed Dec 12, 2020

It's useful to annotate a non-exported top-level function like `g` in

```hs
module Lib (h) where

g :: Int -> Int -> (Int,Int)
g m 1 = (m, 0)
g m n = (2 * m, 2 `div` n)
{-# NOINLINE g #-}

h :: Int -> Int
h 1 = 0
h m
  | odd m     = snd (g m 2)
  | otherwise = uncurry (+) (g 2 m)
```

with its demand `UCU(CS(P(1P(U),SP(U))`, which tells us that whenever `g` was
called, the second component of the returned pair was evaluated strictly.

Since #18903 we do so for local functions, where we can see all calls.
For top-level functions, we can assume that all *exported* functions are
demanded according to `topDmd` and thus get sound demands for
non-exported top-level functions.

The demand on `g` is crucial information for Nested CPR, which may the
go on and unbox `g` for the second pair component. That is true even if
that pair component may diverge, as is the case for the call site `g 13
0`, which throws a div-by-zero exception.

In `T18894b`, you can even see the new demand annotation enabling us to
eta-expand a function that we wouldn't be able to eta-expand without
Call Arity.

We only track bindings of function type in order not to risk huge compile-time
regressions, see `isInterestingTopLevelFn`.

There was a CoreLint check that rejected strict demand annotations on
recursive or top-level bindings, which seems completely unjustified.
All the cases I investigated were fine, so I removed it.

Fixes #18894.