Abortion of fixed-point iteration in Demand Analyser discards sound results

Consider the following program:

-- Extracted from T4903
module Foo where

data Tree = Bin Tree Tree

tree :: Tree
tree = Bin tree tree

eq :: Tree -> Bool
eq (Bin l r) = eq l && eq r

eq amounts to the following Core:

eq
  = \ (ds_d20O :: Tree) ->
      case ds_d20O of { Bin l_aY8 r_aY9 ->
      case eq l_aY8 of {
        False -> False;
        True -> eq r_aY9
      }
      }

It clearly diverges. That's also what pure strictness/termination/CPR analysis would find out. But because usage analysis can't find out in finite time (and that's fine) that eq uses its argument completely, we abort fixed-point iteration [1] and return nopSig, discarding useful and sound strictness information we found out along the way, like the fact that it diverges.

This isn't hard to fix: Just track which parts of the signature were still unsound during abortion and only zap them. I'm only recording this for posterity and as an argument in a discussion I'll maybe start in a few months of time...

[1] This is the ascending chain of demands on ds_d20O during fixed-point iteration:

H
U(1*H,1*H)
U(1*U(1*H,1*H),1*U(1*H,1*H))
...

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Trac field	Value
Version	8.6.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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