Add an ambient Id substitution to Substwip/T10527-2

After a struggle, I fixed Trac #5113 (again) on the 7.10 branch,
by adding an ambient substitution to Subst; see CoreSubst,
esp Note [IdSubstEnv].

This allowed me to do the impedence-matching in SimplEnv.substExpr
efficiently (fixing #10370) as well correctly (fixing the latest
problem with #5113).

This cost me more time than I like to say.  Sigh.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/simplCore/Simplify.hs b/compiler/simplCore/Simplify.hs
index 2e1dcefbdb..40a68d4e6c 100644
--- a/compiler/simplCore/Simplify.hs
+++ b/compiler/simplCore/Simplify.hs
@@ -1179,7 +1179,7 @@ simplCast env body co0 cont0
                 -- But it isn't a common case.
                 --
                 -- Example of use: Trac #995
-         = do { let arg' = substExprS arg_se arg
+         = do { let arg' = substExpr arg_se arg
                            -- It's important that this is lazy, because this argument
                            -- may be disarded if turns out to be the argument of
                            -- (\_ -> e)     This can make a huge difference;