Fix magicDict in ghci (and in the presence of other ticks)

The problem was that ghci inserts some ticks around the crucial bit of
the expression. Just like in some of the other rules we now strip the
ticks so that the rule fires more reliably.

It was possible to defeat magicDict by using -fhpc as well, so not just an
issue in ghci.

Fixes #19667 and related to #19673

(cherry picked from commit 6974c9e478120f6c4eeb53ebfa935c30cafcdf8e)

9 files changed, 71 insertions, 2 deletions

diff --git a/compiler/GHC/Core/Opt/ConstantFold.hs b/compiler/GHC/Core/Opt/ConstantFold.hs
index c34996b507..0826c8c7e1 100644
--- a/compiler/GHC/Core/Opt/ConstantFold.hs
+++ b/compiler/GHC/Core/Opt/ConstantFold.hs
@@ -53,7 +53,7 @@ import GHC.Core.TyCon
    , tyConFamilySize )
 import GHC.Core.DataCon ( dataConTagZ, dataConTyCon, dataConWrapId, dataConWorkId )
 import GHC.Core.Utils  ( eqExpr, cheapEqExpr, exprIsHNF, exprType
-                       , stripTicksTop, stripTicksTopT, mkTicks )
+                       , stripTicksTop, stripTicksTopT, mkTicks, stripTicksE )
 import GHC.Core.Multiplicity
 import GHC.Core.FVs
 import GHC.Core.Type
@@ -2188,7 +2188,7 @@ match_inline _ = Nothing
 -- See Note [magicDictId magic] in "GHC.Types.Id.Make"
 -- for a description of what is going on here.
 match_magicDict :: [Expr CoreBndr] -> Maybe (Expr CoreBndr)
-match_magicDict [Type _, Var wrap `App` Type a `App` Type _ `App` f, x, y ]
+match_magicDict [Type _, (stripTicksE (const True) -> (Var wrap `App` Type a `App` Type _ `App` f)), x, y ]
   | Just (_, fieldTy, _)  <- splitFunTy_maybe $ dropForAlls $ idType wrap
   , Just (_, dictTy, _)   <- splitFunTy_maybe fieldTy
   , Just dictTc           <- tyConAppTyCon_maybe dictTy
diff --git a/compiler/GHC/Types/Id/Make.hs b/compiler/GHC/Types/Id/Make.hs
index 36a2c9d1df..2fe11c66a7 100644
--- a/compiler/GHC/Types/Id/Make.hs
+++ b/compiler/GHC/Types/Id/Make.hs
@@ -1843,6 +1843,11 @@ definition in Core.  The rewrite rule works as follows:
 The `co` coercion is the newtype-coercion extracted from the type-class.
 The type class is obtained by looking at the type of wrap.
 
+In the constant folding rule it's very import to make sure to strip all ticks
+from the expression as if there's an occurence of
+magicDict we *must* convert it for correctness. See #19667 for where this went
+wrong in GHCi.
+
 
 -------------------------------------------------------------
 @realWorld#@ used to be a magic literal, \tr{void#}.  If things get
diff --git a/testsuite/tests/ghci/scripts/T19667Ghci.hs b/testsuite/tests/ghci/scripts/T19667Ghci.hs
new file mode 100644
index 0000000000..21e484bee9
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T19667Ghci.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
+module Main (main) where
+
+import Data.Proxy (Proxy(..))
+import GHC.Exts (magicDict)
+import GHC.TypeLits (Symbol)
+import GHC.Exts
+
+newtype SSymbol (s :: Symbol) = SSymbol String
+
+class KnownSymbol (n :: Symbol) where
+  symbolSing :: SSymbol n
+
+symbolVal :: forall n proxy . KnownSymbol n => proxy n -> String
+symbolVal _ = case symbolSing :: SSymbol n of SSymbol x -> x
+
+data WrapS a b = WrapS (KnownSymbol a => Proxy a -> b)
+
+-- See Note [NOINLINE someNatVal] in GHC.TypeNats
+{-# NOINLINE reifySymbol #-}
+reifySymbol :: forall r. String -> (forall (n :: Symbol). KnownSymbol n => Proxy n -> r) -> r
+reifySymbol n k = magicDict (WrapS k) (SSymbol n) (Proxy @(Any @Symbol))
+
+main :: IO ()
+main = print $ reifySymbol "Hello World" symbolVal
diff --git a/testsuite/tests/ghci/scripts/T19667Ghci.script b/testsuite/tests/ghci/scripts/T19667Ghci.script
new file mode 100644
index 0000000000..5ecc976eaa
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T19667Ghci.script
@@ -0,0 +1,2 @@
+:load T19667Ghci.hs
+:main
diff --git a/testsuite/tests/ghci/scripts/T19667Ghci.stdout b/testsuite/tests/ghci/scripts/T19667Ghci.stdout
new file mode 100644
index 0000000000..06ae699f22
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T19667Ghci.stdout
@@ -0,0 +1 @@
+"Hello World"
diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T
index 0502b1e5a7..bc8de5e5a0 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -328,4 +328,5 @@ test('T19197', normal, ghci_script, ['T19197.script'])
 test('T19158', normal, ghci_script, ['T19158.script'])
 test('T19279', normal, ghci_script, ['T19279.script'])
 test('T19310', normal, ghci_script, ['T19310.script'])
+test('T19667Ghci', extra_files(['T19667Ghci.hs']), ghci_script, ['T19667Ghci.script'])
 test('T19688', normal, ghci_script, ['T19688.script'])
diff --git a/testsuite/tests/typecheck/should_run/T19667.hs b/testsuite/tests/typecheck/should_run/T19667.hs
new file mode 100644
index 0000000000..21e484bee9
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T19667.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
+module Main (main) where
+
+import Data.Proxy (Proxy(..))
+import GHC.Exts (magicDict)
+import GHC.TypeLits (Symbol)
+import GHC.Exts
+
+newtype SSymbol (s :: Symbol) = SSymbol String
+
+class KnownSymbol (n :: Symbol) where
+  symbolSing :: SSymbol n
+
+symbolVal :: forall n proxy . KnownSymbol n => proxy n -> String
+symbolVal _ = case symbolSing :: SSymbol n of SSymbol x -> x
+
+data WrapS a b = WrapS (KnownSymbol a => Proxy a -> b)
+
+-- See Note [NOINLINE someNatVal] in GHC.TypeNats
+{-# NOINLINE reifySymbol #-}
+reifySymbol :: forall r. String -> (forall (n :: Symbol). KnownSymbol n => Proxy n -> r) -> r
+reifySymbol n k = magicDict (WrapS k) (SSymbol n) (Proxy @(Any @Symbol))
+
+main :: IO ()
+main = print $ reifySymbol "Hello World" symbolVal
diff --git a/testsuite/tests/typecheck/should_run/T19667.stdout b/testsuite/tests/typecheck/should_run/T19667.stdout
new file mode 100644
index 0000000000..06ae699f22
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T19667.stdout
@@ -0,0 +1 @@
+"Hello World"
diff --git a/testsuite/tests/typecheck/should_run/all.T b/testsuite/tests/typecheck/should_run/all.T
index c4005e402e..efc9fcb374 100755
--- a/testsuite/tests/typecheck/should_run/all.T
+++ b/testsuite/tests/typecheck/should_run/all.T
@@ -155,3 +155,4 @@ test('T19397M1', extra_files(['T19397S.hs']), compile_and_run, [''])
 test('T19397M2', extra_files(['T19397S.hs']), compile_and_run, ['-main-is foo'])
 test('T19397M3', extra_files(['T19397S.hs']), compile_and_run, ['-main-is foo'])
 test('T19397M4', extra_files(['T19397S.hs']), compile_and_run, ['-main-is foo'])
+test('T19667', normal, compile_and_run, ['-fhpc'])