Warn a bit less often about unlifted bindings.

Warn when
   (a) a pattern bindings binds unlifted values
   (b) it has no top-level bang
   (c) the RHS has a *lifted* type

Clause (c) is new, argued for by Simon M

Eg     x# = 4# + 4#      -- No warning
       (# a,b #) = blah  -- No warning
       I# x = blah       -- Warning

compiler/hsSyn/HsPat.lhs

@@ -22,7 +22,8 @@ module HsPat (
 
 	mkPrefixConPat, mkCharLitPat, mkNilPat, 
 
-	isBangHsBind, isBangLPat, hsPatNeedsParens,
+        isBangHsBind, isLiftedPatBind,
+        isBangLPat, hsPatNeedsParens,
 	isIrrefutableHsPat,
 
 	pprParendLPat
@@ -374,10 +375,29 @@ isBangLPat (L _ (ParPat p))   = isBangLPat p
 isBangLPat _                  = False
 
 isBangHsBind :: HsBind id -> Bool
--- In this module because HsPat is above HsBinds in the import graph
+-- A pattern binding with an outermost bang
+-- Defined in this module because HsPat is above HsBinds in the import graph
 isBangHsBind (PatBind { pat_lhs = p }) = isBangLPat p
 isBangHsBind _                         = False
 
+isLiftedPatBind :: HsBind id -> Bool
+-- A pattern binding with a compound pattern, not just a variable
+--    (I# x)       yes
+--    (# a, b #)   no, even if a::Int#
+--    x            no, even if x::Int#
+-- We want to warn about a missing bang-pattern on the yes's
+isLiftedPatBind (PatBind { pat_lhs = p }) = isLiftedLPat p
+isLiftedPatBind _                         = False
+
+isLiftedLPat :: LPat id -> Bool
+isLiftedLPat (L _ (ParPat p))   = isLiftedLPat p
+isLiftedLPat (L _ (BangPat p))  = isLiftedLPat p
+isLiftedLPat (L _ (AsPat _ p))  = isLiftedLPat p
+isLiftedLPat (L _ (TuplePat _ Unboxed _)) = False
+isLiftedLPat (L _ (VarPat {}))            = False
+isLiftedLPat (L _ (WildPat {}))           = False
+isLiftedLPat _                            = True
+
 isIrrefutableHsPat :: OutputableBndr id => LPat id -> Bool
 -- (isIrrefutableHsPat p) is true if matching against p cannot fail,
 -- in the sense of falling through to the next pattern.

compiler/typecheck/TcBinds.lhs

@@ -1134,19 +1134,25 @@ checkStrictBinds top_lvl rec_group binds poly_ids
         -- the versions of alex and happy available have non-conforming
         -- templates, so the GHC build fails if it's an error:
         ; warnUnlifted <- doptM Opt_WarnLazyUnliftedBindings
-        ; warnTc (warnUnlifted && not bang_pat)
+        ; warnTc (warnUnlifted && not bang_pat && lifted_pat)
+                 -- No outer bang, but it's a compound pattern
+                 -- E.g   (I# x#) = blah
+                 -- Warn about this, but not about
+                 --      x# = 4# +# 1#
+                 --      (# a, b #) = ...
                  (unliftedMustBeBang binds) }
   | otherwise
   = return ()
   where
-    unlifted = any is_unlifted poly_ids
-    bang_pat = any (isBangHsBind . unLoc) binds
+    unlifted    = any is_unlifted poly_ids
+    bang_pat    = any (isBangHsBind . unLoc) binds
+    lifted_pat  = any (isLiftedPatBind . unLoc) binds
     is_unlifted id = case tcSplitForAllTys (idType id) of
                        (_, rho) -> isUnLiftedType rho
 
 unliftedMustBeBang :: [LHsBind Name] -> SDoc
 unliftedMustBeBang binds
-  = hang (text "Bindings containing unlifted types should use an outermost bang pattern:")
+  = hang (text "Pattern bindings containing unlifted types should use an outermost bang pattern:")
        2 (pprBindList binds)
 
 strictBindErr :: String -> Bool -> [LHsBind Name] -> SDoc