Improve CSE in STG-land

This patch fixes #19717, a long-standing bug in CSE for STG, which
led to a stupid loss of CSE in some situations.

It's explained in Note [Trivial case scrutinee], which I have
substantially extended.

3 files changed, 26 insertions, 0 deletions

diff --git a/testsuite/tests/simplStg/should_compile/T19717.hs b/testsuite/tests/simplStg/should_compile/T19717.hs
new file mode 100644
index 0000000000..2b485b6464
--- /dev/null
+++ b/testsuite/tests/simplStg/should_compile/T19717.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module Foo where
+
+
+f x = x `seq` [Just x, Just x]
diff --git a/testsuite/tests/simplStg/should_compile/T19717.stderr b/testsuite/tests/simplStg/should_compile/T19717.stderr
new file mode 100644
index 0000000000..9dd1e085f8
--- /dev/null
+++ b/testsuite/tests/simplStg/should_compile/T19717.stderr
@@ -0,0 +1,19 @@
+
+==================== Final STG: ====================
+Foo.f :: forall {a}. a -> [GHC.Maybe.Maybe a]
+[GblId, Arity=1, Str=<1L>, Cpr=2, Unf=OtherCon []] =
+    {} \r [x]
+        case x of x1 {
+        __DEFAULT ->
+        let {
+          sat [Occ=Once1] :: GHC.Maybe.Maybe a
+          [LclId] =
+              CCCS GHC.Maybe.Just! [x1]; } in
+        let {
+          sat [Occ=Once1] :: [GHC.Maybe.Maybe a]
+          [LclId] =
+              CCCS :! [sat GHC.Types.[]];
+        } in  : [sat sat];
+        };
+
+
diff --git a/testsuite/tests/simplStg/should_compile/all.T b/testsuite/tests/simplStg/should_compile/all.T
index bb2e25ed4b..8cc4c49922 100644
--- a/testsuite/tests/simplStg/should_compile/all.T
+++ b/testsuite/tests/simplStg/should_compile/all.T
@@ -10,3 +10,4 @@ def f( name, opts ):
 setTestOpts(f)
 
 test('T13588', [ grep_errmsg('case') ] , compile, ['-dverbose-stg2stg -fno-worker-wrapper'])
+test('T19717', normal, compile, ['-ddump-stg-final -dsuppress-uniques -dno-typeable-binds'])