Add mkHsCoerce to avoid junk in typechecked code

[ghc-hetmet.git] / ghc / compiler / deSugar / DsGRHSs.lhs

diff --git a/ghc/compiler/deSugar/DsGRHSs.lhs b/ghc/compiler/deSugar/DsGRHSs.lhs
index 33f86ed..eea61ba 100644 (file)
--- a/ghc/compiler/deSugar/DsGRHSs.lhs
+++ b/ghc/compiler/deSugar/DsGRHSs.lhs
@@ -90,12 +90,16 @@ matchGuards [] ctx rhs rhs_ty
        ; return (cantFailMatchResult core_rhs) }
 
        -- ExprStmts must be guards
-       -- Turn an "otherwise" guard is a no-op
+       -- Turn an "otherwise" guard is a no-op.  This ensures that 
+       -- you don't get a "non-exhaustive eqns" message when the guards 
+       -- finish in "otherwise".
+       -- NB:  The success of this clause depends on the typechecker not
+       --      wrapping the 'otherwise' in empty HsTyApp or HsCoerce constructors
+       --      If it does, you'll get bogus overlap warnings
 matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
   |  v `hasKey` otherwiseIdKey
   || v `hasKey` getUnique trueDataConId        
-       -- trueDataConId doesn't have the same 
-       -- unique as trueDataCon
+       -- trueDataConId doesn't have the same unique as trueDataCon
   = matchGuards stmts ctx rhs rhs_ty
 
 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty