Fix a buglet in Simplify.simplCastwip/T18347

This bug, revealed by #18347, is just a missing update to
sc_hole_ty in simplCast.  I'd missed a code path when I
made the recentchanges in

    commit 6d49d5be904c0c01788fa7aae1b112d5b4dfaf1c
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Thu May 21 12:53:35 2020 +0100

    Implement cast worker/wrapper properly

The fix is very easy.

Two other minor changes

* Tidy up in SimpleOpt.simple_opt_expr. In fact I think this is an
  outright bug, introduced in the fix to #18112: we were simplifying
  the same coercion twice *with the same substitution*, which is just
  wrong.  It'd be a hard bug to trigger, so I just fixed it; less code
  too.

* Better debug printing of ApplyToVal

1 files changed, 15 insertions, 9 deletions

diff --git a/compiler/GHC/Core/SimpleOpt.hs b/compiler/GHC/Core/SimpleOpt.hs
index 87ad9e69c5..9901f752b1 100644
--- a/compiler/GHC/Core/SimpleOpt.hs
+++ b/compiler/GHC/Core/SimpleOpt.hs
@@ -213,6 +213,7 @@ simple_opt_expr env expr
     in_scope     = substInScope subst
     in_scope_env = (in_scope, simpleUnfoldingFun)
 
+    ---------------
     go (Var v)
        | Just clo <- lookupVarEnv (soe_inl env) v
        = simple_opt_clo env clo
@@ -221,17 +222,10 @@ simple_opt_expr env expr
 
     go (App e1 e2)      = simple_app env e1 [(env,e2)]
     go (Type ty)        = Type     (substTy subst ty)
-    go (Coercion co)    = Coercion (optCoercion (soe_dflags env) (getTCvSubst subst) co)
+    go (Coercion co)    = Coercion (go_co co)
     go (Lit lit)        = Lit lit
     go (Tick tickish e) = mkTick (substTickish subst tickish) (go e)
-    go (Cast e co)      = case go e of
-                            -- flatten nested casts before calling the coercion optimizer;
-                            -- see #18112 (note that mkCast handles dropping Refl coercions)
-                            Cast e' co' -> mkCast e' (opt_co (mkTransCo co' co))
-                            e'          -> mkCast e' (opt_co co)
-                          where
-                            opt_co = optCoercion (soe_dflags env) (getTCvSubst subst)
-
+    go (Cast e co)      = mk_cast (go e) (go_co co)
     go (Let bind body)  = case simple_opt_bind env bind NotTopLevel of
                              (env', Nothing)   -> simple_opt_expr env' body
                              (env', Just bind) -> Let bind (simple_opt_expr env' body)
@@ -267,6 +261,9 @@ simple_opt_expr env expr
         (env', b') = subst_opt_bndr env b
 
     ----------------------
+    go_co co = optCoercion (soe_dflags env) (getTCvSubst subst) co
+
+    ----------------------
     go_alt env (con, bndrs, rhs)
       = (con, bndrs', simple_opt_expr env' rhs)
       where
@@ -285,6 +282,15 @@ simple_opt_expr env expr
          bs = reverse bs'
          e' = simple_opt_expr env e
 
+mk_cast :: CoreExpr -> CoercionR -> CoreExpr
+-- Like GHC.Core.Utils.mkCast, but does a full reflexivity check.
+-- mkCast doesn't do that because the Simplifier does (in simplCast)
+-- But in SimpleOpt it's nice to kill those nested casts (#18112)
+mk_cast (Cast e co1) co2        = mk_cast e (co1 `mkTransCo` co2)
+mk_cast (Tick t e)   co         = Tick t (mk_cast e co)
+mk_cast e co | isReflexiveCo co = e
+             | otherwise        = Cast e co
+
 ----------------------
 -- simple_app collects arguments for beta reduction
 simple_app :: SimpleOptEnv -> InExpr -> [SimpleClo] -> CoreExpr