Improve exprOkForSpeculation for classopswip/T22745

This patch fixes #22745 and #15205, which are about GHC's
failure to discard unnecessary superclass selections that
yield coercions.  See
  GHC.Core.Utils Note [exprOkForSpeculation and type classes]

The main changes are:

* Write new Note [NON-BOTTOM_DICTS invariant] in GHC.Core, and
  refer to it

* Define new function isTerminatingType, to identify those
  guaranteed-terminating dictionary types.

* exprOkForSpeculation has a new (very simple) case for ClassOpId

* ClassOpId has a new field that says if the return type is
  an unlifted type, or a terminating type.

This was surprisingly tricky to get right.  In particular note
that unlifted types are not terminating types; you can write an
expression of unlifted type, that diverges.  Not so for dictionaries
(or, more precisely, for the dictionaries that GHC constructs).

Metric Decrease:
    LargeRecord

1 files changed, 7 insertions, 3 deletions

diff --git a/compiler/GHC/CoreToStg/Prep.hs b/compiler/GHC/CoreToStg/Prep.hs
index 78ce8e16f1..c0b72cefed 100644
--- a/compiler/GHC/CoreToStg/Prep.hs
+++ b/compiler/GHC/CoreToStg/Prep.hs
@@ -1648,12 +1648,16 @@ long as the callee might evaluate it. And if it is evaluated on
 most code paths anyway, we get to turn the unknown eval in the
 callee into a known call at the call site.
 
-However, we must be very careful not to speculate recursive calls!
-Doing so might well change termination behavior.
+Very Nasty Wrinkle
+
+We must be very careful not to speculate recursive calls!  Doing so
+might well change termination behavior.
 
 That comes up in practice for DFuns, which are considered ok-for-spec,
 because they always immediately return a constructor.
-Not so if you speculate the recursive call, as #20836 shows:
+See Note [NON-BOTTOM-DICTS invariant] in GHC.Core.
+
+But not so if you speculate the recursive call, as #20836 shows:
 
   class Foo m => Foo m where
     runFoo :: m a -> m a