Define DsUtils.mkCastDs and use it

This change avoids a spurious WARNing from mkCast.  In the output of
the desugarer (only, I think) we can have a cast where the type of the
expression and cast don't syntactically match, because of an enclosing
type-let binding.

1 files changed, 23 insertions, 4 deletions

diff --git a/compiler/deSugar/DsUtils.hs b/compiler/deSugar/DsUtils.hs
index f94b831a6f..819944312b 100644
--- a/compiler/deSugar/DsUtils.hs
+++ b/compiler/deSugar/DsUtils.hs
@@ -24,7 +24,7 @@ module DsUtils (
         mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult, mkCoSynCaseMatchResult,
         wrapBind, wrapBinds,
 
-        mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs,
+        mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs, mkCastDs,
 
         seqVar,
 
@@ -44,6 +44,7 @@ import {-# SOURCE #-}   Match ( matchSimply )
 
 import HsSyn
 import TcHsSyn
+import Coercion( Coercion, isReflCo )
 import TcType( tcSplitTyConApp )
 import CoreSyn
 import DsMonad
@@ -549,10 +550,22 @@ mkCoreAppDs fun arg = mkCoreApp fun arg  -- The rest is done in MkCore
 mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
 mkCoreAppsDs fun args = foldl mkCoreAppDs fun args
 
+mkCastDs :: CoreExpr -> Coercion -> CoreExpr
+-- We define a desugarer-specific verison of CoreUtils.mkCast,
+-- because in the immediate output of the desugarer, we can have
+-- apparently-mis-matched coercions:  E.g.
+--     let a = b
+--     in (x :: a) |> (co :: b ~ Int)
+-- Lint know about type-bindings for let and does not complain
+-- So here we do not make the assertion checks that we make in
+-- CoreUtils.mkCast; and we do less peephole optimisation too
+mkCastDs e co | isReflCo co = e
+              | otherwise   = Cast e co
+
 {-
 ************************************************************************
 *                                                                      *
-\subsection[mkSelectorBind]{Make a selector bind}
+               Tuples and selector bindings
 *                                                                      *
 ************************************************************************
 
@@ -720,7 +733,7 @@ mkBigLHsPatTup = mkChunkified mkLHsPatTup
 {-
 ************************************************************************
 *                                                                      *
-\subsection[mkFailurePair]{Code for pattern-matching and other failures}
+        Code for pattern-matching and other failures
 *                                                                      *
 ************************************************************************
 
@@ -805,7 +818,13 @@ entered at most once.  Adding a dummy 'realWorld' token argument makes
 it clear that sharing is not an issue.  And that in turn makes it more
 CPR-friendly.  This matters a lot: if you don't get it right, you lose
 the tail call property.  For example, see Trac #3403.
--}
+
+
+************************************************************************
+*                                                                      *
+              Ticks
+*                                                                      *
+********************************************************************* -}
 
 mkOptTickBox :: [Tickish Id] -> CoreExpr -> CoreExpr
 mkOptTickBox = flip (foldr Tick)