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, 72 insertions, 2 deletions

diff --git a/compiler/GHC/Core/Opt/ConstantFold.hs b/compiler/GHC/Core/Opt/ConstantFold.hs
index 1ca5fbe52f..55b262d0d1 100644
--- a/compiler/GHC/Core/Opt/ConstantFold.hs
+++ b/compiler/GHC/Core/Opt/ConstantFold.hs
@@ -44,7 +44,7 @@ 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.Unfold ( exprIsConApp_maybe )
 import GHC.Core.Multiplicity
 import GHC.Core.FVs
@@ -1832,7 +1832,7 @@ 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 f40d723d7a..9e6bcd196e 100644
--- a/compiler/GHC/Types/Id/Make.hs
+++ b/compiler/GHC/Types/Id/Make.hs
@@ -1733,6 +1733,11 @@ 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 obtain 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 5e34df6041..77b7f127c9 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -319,3 +319,5 @@ test('T17431', normal, ghci_script, ['T17431.script'])
 test('T17549', normal, ghci_script, ['T17549.script'])
 test('T17669', [extra_run_opts('-fexternal-interpreter -fobject-code'), expect_broken(17669)], ghci_script, ['T17669.script'])
 test('T18955', [extra_hc_opts("-fobject-code")], ghci_script, ['T18955.script'])
+test('T19667Ghci', extra_files(['T19667Ghci.hs']), ghci_script, ['T19667Ghci.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 7468511779..db37c4a314 100755
--- a/testsuite/tests/typecheck/should_run/all.T
+++ b/testsuite/tests/typecheck/should_run/all.T
@@ -154,3 +154,4 @@ 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'])