Do not mark CoVars as dead in the occur-anal

For years we have been marking CoVars as dead, becuase we
don't gather occurrence info from types.  This is obviously
wrong and caused Trac #15695.

See Note [Do not mark CoVars as dead] in OccurAnal.

1 files changed, 23 insertions, 0 deletions

diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index 236bb81066..2cc38a27e0 100644
--- a/compiler/simplCore/OccurAnal.hs
+++ b/compiler/simplCore/OccurAnal.hs
@@ -2501,6 +2501,10 @@ zapDetails = markAllMany . markAllNonTailCalled -- effectively sets to noOccInfo
 
 lookupDetails :: UsageDetails -> Id -> OccInfo
 lookupDetails ud id
+  | isCoVar id  -- We do not currenly gather occurrence info (from types)
+  = noOccInfo   -- for CoVars, so we must conservatively mark them as used
+                -- See Note [DoO not mark CoVars as dead]
+  | otherwise
   = case lookupVarEnv (ud_env ud) id of
       Just occ -> doZapping ud id occ
       Nothing  -> IAmDead
@@ -2512,6 +2516,25 @@ udFreeVars :: VarSet -> UsageDetails -> VarSet
 -- Find the subset of bndrs that are mentioned in uds
 udFreeVars bndrs ud = restrictUniqSetToUFM bndrs (ud_env ud)
 
+{- Note [Do not mark CoVars as dead]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's obviously wrong to mark CoVars as dead if they are used.
+Currently we don't traverse types to gather usase info for CoVars,
+so we had better treat them as having noOccInfo.
+
+This showed up in Trac #15696 we had something like
+  case eq_sel d of co -> ...(typeError @(...co...) "urk")...
+
+Then 'd' was substitued by a dictionary, so the expression
+simpified to
+  case (Coercion <blah>) of co -> ...(typeError @(...co...) "urk")...
+
+But then the "drop the case altogether" equation of rebuildCase
+thought that 'co' was dead, and discarded the entire case. Urk!
+
+I have no idea how we managed to avoid this pitfall for so long!
+-}
+
 -------------------
 -- Auxiliary functions for UsageDetails implementation