Detect levity-polymorphic uses of unsafeCoerce#

This bug was shown up by Trac #14561. The deguarer carefully
detects unsaturated and levity-polymorphic uses of primops,
but not of things like unsafeCoerce#.

The fix is simple: see Note [Levity-polymorphic Ids] in Id.

1 files changed, 23 insertions, 3 deletions

diff --git a/compiler/basicTypes/Id.hs b/compiler/basicTypes/Id.hs
index 63ca38cb61..fbece0e68d 100644
--- a/compiler/basicTypes/Id.hs
+++ b/compiler/basicTypes/Id.hs
@@ -119,7 +119,8 @@ module Id (
 import GhcPrelude
 
 import DynFlags
-import CoreSyn ( CoreRule, isStableUnfolding, evaldUnfolding, Unfolding( NoUnfolding ) )
+import CoreSyn ( CoreRule, isStableUnfolding, evaldUnfolding,
+                 isCompulsoryUnfolding, Unfolding( NoUnfolding ) )
 
 import IdInfo
 import BasicTypes
@@ -519,7 +520,8 @@ hasNoBinding id = case Var.idDetails id of
                         PrimOpId _       -> True        -- See Note [Primop wrappers]
                         FCallId _        -> True
                         DataConWorkId dc -> isUnboxedTupleCon dc || isUnboxedSumCon dc
-                        _                -> False
+                        _                -> isCompulsoryUnfolding (idUnfolding id)
+                                            -- See Note [Levity-polymorphic Ids]
 
 isImplicitId :: Id -> Bool
 -- ^ 'isImplicitId' tells whether an 'Id's info is implied by other
@@ -541,7 +543,25 @@ isImplicitId id
 idIsFrom :: Module -> Id -> Bool
 idIsFrom mod id = nameIsLocalOrFrom mod (idName id)
 
-{-
+{- Note [Levity-polymorphic Ids]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Some levity-polymorphic Ids must be applied and and inlined, not left
+un-saturated.  Example:
+  unsafeCoerceId :: forall r1 r2 (a::TYPE r1) (b::TYPE r2). a -> b
+
+This has a compulsory unfolding because we can't lambda-bind those
+arguments.  But the compulsory unfolding may leave levity-polymorphic
+lambdas if it is not applied to enough arguments; e.g. (Trac #14561)
+  bad :: forall (a :: TYPE r). a -> a
+  bad = unsafeCoerce#
+
+The desugar has special magic to detect such cases: DsExpr.badUseOfLevPolyPrimop.
+And we want that magic to apply to levity-polymorphic compulsory-inline things.
+The easiest way to do this is for hasNoBinding to return True of all things
+that have compulsory unfolding.  A very Ids with a compulsory unfolding also
+have a binding, but it does not harm to say they don't here, and its a very
+simple way to fix Trac #14561.
+
 Note [Primop wrappers]
 ~~~~~~~~~~~~~~~~~~~~~~
 Currently hasNoBinding claims that PrimOpIds don't have a curried