Do not treat a constructor in a *pattern* as a *use* of that constructor

Occurrences in terms are uses, in patterns they are not.
In this way we get unused-constructor warnings from modules like this

   module M( f, g, T ) where
     data T = T1 | T2 Bool

     f x = T2 x

     g T1 = True
     g (T2 x) = x

Here a T1 value cannot be constructed, so we can warn. The use
in a pattern doesn't count.  See Note [Patterns are not uses]
in RnPat.

Interestingly this change exposed three module in GHC itself
that had unused constructors, which I duly removed:
  * ghc/Main.hs
  * compiler/ghci/ByteCodeAsm
  * compiler/nativeGen/PPC/RegInfo
Their changes are in this patch.

1 files changed, 18 insertions, 2 deletions

diff --git a/compiler/rename/RnPat.lhs b/compiler/rename/RnPat.lhs
index c3b40fe0f2..9738585aa4 100644
--- a/compiler/rename/RnPat.lhs
+++ b/compiler/rename/RnPat.lhs
@@ -121,10 +121,26 @@ wrapSrcSpanCps fn (L loc a)
 lookupConCps :: Located RdrName -> CpsRn (Located Name)
 lookupConCps con_rdr 
   = CpsRn (\k -> do { con_name <- lookupLocatedOccRn con_rdr
-                    ; (r, fvs) <- k con_name
-                    ; return (r, fvs `plusFV` unitFV (unLoc con_name)) })
+                    ; k con_name })
+    -- We do not add the constructor name to the free vars
+    -- See Note [Patterns are not uses]
 \end{code}
 
+Note [Patterns are not uses]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+  module Foo( f, g ) where
+  data T = T1 | T2
+
+  f T1 = True
+  f T2 = False
+
+  g _ = T1
+
+Arguaby we should report T2 as unused, even though it appears in a
+pattern, because it never occurs in a constructed position.  See
+Trac #7336.
+
 %*********************************************************
 %*							*
 	Name makers