Export (~) from Data.Type.Equality (#18862)wip/eqtycon-rn

* Users can define their own (~) type operator
* Haddock can display documentation for the built-in (~)
* New transitional warnings implemented:
    -Wtype-equality-out-of-scope
    -Wtype-equality-requires-operators

Updates the haddock submodule.

68 files changed, 277 insertions, 94 deletions

diff --git a/compiler/GHC/Builtin/Names.hs b/compiler/GHC/Builtin/Names.hs
index 9b1c9bad01..e17f2dda44 100644
--- a/compiler/GHC/Builtin/Names.hs
+++ b/compiler/GHC/Builtin/Names.hs
@@ -2770,4 +2770,5 @@ pretendNameIsInScope n
     [ liftedTypeKindTyConKey, unliftedTypeKindTyConKey
     , liftedDataConKey, unliftedDataConKey
     , tYPETyConKey
-    , runtimeRepTyConKey, boxedRepDataConKey ]
+    , runtimeRepTyConKey, boxedRepDataConKey
+    , eqTyConKey ]
diff --git a/compiler/GHC/Builtin/Types.hs b/compiler/GHC/Builtin/Types.hs
index 347afad5c0..54354fcd5f 100644
--- a/compiler/GHC/Builtin/Types.hs
+++ b/compiler/GHC/Builtin/Types.hs
@@ -343,27 +343,6 @@ eqTyConName   = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "~")   eqTyConK
 eqDataConName = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "Eq#") eqDataConKey eqDataCon
 eqSCSelIdName = mkWiredInIdName gHC_TYPES (fsLit "eq_sel") eqSCSelIdKey eqSCSelId
 
-{- Note [eqTyCon (~) is built-in syntax]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The (~) type operator used in equality constraints (a~b) is considered built-in
-syntax. This has a few consequences:
-
-* The user is not allowed to define their own type constructors with this name:
-
-    ghci> class a ~ b
-    <interactive>:1:1: error: Illegal binding of built-in syntax: ~
-
-* Writing (a ~ b) does not require enabling -XTypeOperators. It does, however,
-  require -XGADTs or -XTypeFamilies.
-
-* The (~) type operator is always in scope. It doesn't need to be imported,
-  and it cannot be hidden.
-
-* We have a bunch of special cases in the compiler to arrange all of the above.
-
-There's no particular reason for (~) to be special, but fixing this would be a
-breaking change.
--}
 eqTyCon_RDR :: RdrName
 eqTyCon_RDR = nameRdrName eqTyConName
 
@@ -893,9 +872,6 @@ isBuiltInOcc_maybe occ =
       "[]" -> Just $ choose_ns listTyConName nilDataConName
       ":"    -> Just consDataConName
 
-      -- equality tycon
-      "~"    -> Just eqTyConName
-
       -- function tycon
       "FUN"  -> Just funTyConName
       "->"  -> Just unrestrictedFunTyConName
diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index 21649c9c54..1d1222fbab 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -553,6 +553,8 @@ data WarningFlag =
    | Opt_WarnForallIdentifier                        -- Since 9.4
    | Opt_WarnUnicodeBidirectionalFormatCharacters    -- Since 9.0.2
    | Opt_WarnGADTMonoLocalBinds                      -- Since 9.4
+   | Opt_WarnTypeEqualityOutOfScope                  -- Since 9.4
+   | Opt_WarnTypeEqualityRequiresOperators           -- Since 9.4
    deriving (Eq, Ord, Show, Enum)
 
 -- | Return the names of a WarningFlag
@@ -655,6 +657,8 @@ warnFlagNames wflag = case wflag of
   Opt_WarnForallIdentifier                        -> "forall-identifier" :| []
   Opt_WarnUnicodeBidirectionalFormatCharacters    -> "unicode-bidirectional-format-characters" :| []
   Opt_WarnGADTMonoLocalBinds                      -> "gadt-mono-local-binds" :| []
+  Opt_WarnTypeEqualityOutOfScope                  -> "type-equality-out-of-scope" :| []
+  Opt_WarnTypeEqualityRequiresOperators           -> "type-equality-requires-operators" :| []
 
 -- -----------------------------------------------------------------------------
 -- Standard sets of warning options
@@ -748,7 +752,8 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnOperatorWhitespaceExtConflict,
         Opt_WarnForallIdentifier,
         Opt_WarnUnicodeBidirectionalFormatCharacters,
-        Opt_WarnGADTMonoLocalBinds
+        Opt_WarnGADTMonoLocalBinds,
+        Opt_WarnTypeEqualityRequiresOperators
       ]
 
 -- | Things you get with -W
@@ -801,6 +806,7 @@ minusWcompatOpts
       , Opt_WarnNonCanonicalMonoidInstances
       , Opt_WarnStarIsType
       , Opt_WarnCompatUnqualifiedImports
+      , Opt_WarnTypeEqualityOutOfScope
       ]
 
 -- | Things you get with -Wunused-binds
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 49e322bbd2..d62660c519 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3295,7 +3295,9 @@ wWarningFlagsDeps = mconcat [
   warnSpec    Opt_WarnMissingExportedPatternSynonymSignatures,
   warnSpec    Opt_WarnForallIdentifier,
   warnSpec    Opt_WarnUnicodeBidirectionalFormatCharacters,
-  warnSpec    Opt_WarnGADTMonoLocalBinds
+  warnSpec    Opt_WarnGADTMonoLocalBinds,
+  warnSpec    Opt_WarnTypeEqualityOutOfScope,
+  warnSpec    Opt_WarnTypeEqualityRequiresOperators
  ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
diff --git a/compiler/GHC/Parser.y b/compiler/GHC/Parser.y
index a11a438d89..24ef753453 100644
--- a/compiler/GHC/Parser.y
+++ b/compiler/GHC/Parser.y
@@ -90,7 +90,7 @@ import GHC.Parser.Errors.Ppr ()
 import GHC.Builtin.Types ( unitTyCon, unitDataCon, sumTyCon,
                            tupleTyCon, tupleDataCon, nilDataCon,
                            unboxedUnitTyCon, unboxedUnitDataCon,
-                           listTyCon_RDR, consDataCon_RDR, eqTyCon_RDR)
+                           listTyCon_RDR, consDataCon_RDR)
 
 import qualified Data.Semigroup as Semi
 }
@@ -3641,11 +3641,7 @@ qtyconsym :: { LocatedN RdrName }
 
 tyconsym :: { LocatedN RdrName }
         : CONSYM                { sL1n $1 $! mkUnqual tcClsName (getCONSYM $1) }
-        | VARSYM                { sL1n $1 $!
-                                    -- See Note [eqTyCon (~) is built-in syntax] in GHC.Builtin.Types
-                                    if getVARSYM $1 == fsLit "~"
-                                      then eqTyCon_RDR
-                                      else mkUnqual tcClsName (getVARSYM $1) }
+        | VARSYM                { sL1n $1 $! mkUnqual tcClsName (getVARSYM $1) }
         | ':'                   { sL1n $1 $! consDataCon_RDR }
         | '-'                   { sL1n $1 $! mkUnqual tcClsName (fsLit "-") }
         | '.'                   { sL1n $1 $! mkUnqual tcClsName (fsLit ".") }
diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index e5557855c0..444471abca 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -137,7 +137,7 @@ import GHC.Types.TyThing
 import GHC.Core.Type    ( unrestrictedFunTyCon, Specificity(..) )
 import GHC.Builtin.Types( cTupleTyConName, tupleTyCon, tupleDataCon,
                           nilDataConName, nilDataConKey,
-                          listTyConName, listTyConKey, eqTyCon_RDR )
+                          listTyConName, listTyConKey )
 import GHC.Types.ForeignCall
 import GHC.Types.SrcLoc
 import GHC.Types.Unique ( hasKey )
@@ -2453,8 +2453,7 @@ checkPrecP (L l (_,i)) (L _ ol)
  | otherwise = addFatalError $ mkPlainErrorMsgEnvelope l (PsErrPrecedenceOutOfRange i)
   where
     -- If you change this, consider updating Note [Fixity of (->)] in GHC/Types.hs
-    specialOp op = unLoc op `elem` [ eqTyCon_RDR
-                                   , getRdrName unrestrictedFunTyCon ]
+    specialOp op = unLoc op == getRdrName unrestrictedFunTyCon
 
 mkRecConstrOrUpdate
         :: Bool
diff --git a/compiler/GHC/Rename/Env.hs b/compiler/GHC/Rename/Env.hs
index cd40ab100a..3525c71f1b 100644
--- a/compiler/GHC/Rename/Env.hs
+++ b/compiler/GHC/Rename/Env.hs
@@ -1047,7 +1047,24 @@ lookupTypeOccRn rdr_name
   = do { mb_name <- lookupOccRn_maybe rdr_name
        ; case mb_name of
              Just name -> return name
-             Nothing   -> lookup_demoted rdr_name }
+             Nothing   ->
+               if occName rdr_name == occName eqTyCon_RDR -- See Note [eqTyCon (~) compatibility fallback]
+               then eqTyConName <$ addDiagnostic TcRnTypeEqualityOutOfScope
+               else lookup_demoted rdr_name }
+
+{- Note [eqTyCon (~) compatibility fallback]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Before GHC Proposal #371, the (~) type operator used in type equality
+constraints (a~b) was considered built-in syntax.
+
+This had two implications:
+
+1. Users could use it without importing it from Data.Type.Equality or Prelude.
+2. TypeOperators were not required to use it (it was guarded behind TypeFamilies/GADTs instead)
+
+To ease migration and minimize breakage, we continue to support those usages
+but emit appropriate warnings.
+-}
 
 lookup_demoted :: RdrName -> RnM Name
 lookup_demoted rdr_name
@@ -1919,13 +1936,7 @@ dataTcOccs rdr_name
   = [rdr_name]
   where
     occ = rdrNameOcc rdr_name
-    rdr_name_tc =
-      case rdr_name of
-        -- The (~) type operator is always in scope, so we need a special case
-        -- for it here, or else  :info (~)  fails in GHCi.
-        -- See Note [eqTyCon (~) is built-in syntax]
-        Unqual occ | occNameFS occ == fsLit "~" -> eqTyCon_RDR
-        _ -> setRdrNameSpace rdr_name tcName
+    rdr_name_tc = setRdrNameSpace rdr_name tcName
 
 {-
 Note [dataTcOccs and Exact Names]
diff --git a/compiler/GHC/Rename/Expr.hs-boot b/compiler/GHC/Rename/Expr.hs-boot
index 58f6bbc874..ca66c8168b 100644
--- a/compiler/GHC/Rename/Expr.hs-boot
+++ b/compiler/GHC/Rename/Expr.hs-boot
@@ -1,6 +1,12 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE ConstraintKinds #-}
 module GHC.Rename.Expr where
+
+#if MIN_VERSION_GLASGOW_HASKELL(9,3,0,0)
+import Data.Type.Equality (type (~))
+#endif
+
 import GHC.Types.Name
 import GHC.Hs
 import GHC.Types.Name.Set ( FreeVars )
diff --git a/compiler/GHC/Rename/HsType.hs b/compiler/GHC/Rename/HsType.hs
index bf31991e8f..edda60fbee 100644
--- a/compiler/GHC/Rename/HsType.hs
+++ b/compiler/GHC/Rename/HsType.hs
@@ -629,7 +629,7 @@ rnHsTyKi env (HsTyVar _ ip (L loc rdr_name))
 
 rnHsTyKi env ty@(HsOpTy _ ty1 l_op ty2)
   = setSrcSpan (getLocA l_op) $
-    do  { (l_op', fvs1) <- rnHsTyOp env ty l_op
+    do  { (l_op', fvs1) <- rnHsTyOp env (ppr ty) l_op
         ; fix   <- lookupTyFixityRn l_op'
         ; (ty1', fvs2) <- rnLHsTyKi env ty1
         ; (ty2', fvs3) <- rnLHsTyKi env ty2
@@ -822,16 +822,15 @@ rnLTyVar (L loc rdr_name)
        ; return (L loc tyvar) }
 
 --------------
-rnHsTyOp :: Outputable a
-         => RnTyKiEnv -> a -> LocatedN RdrName
+rnHsTyOp :: RnTyKiEnv -> SDoc -> LocatedN RdrName
          -> RnM (LocatedN Name, FreeVars)
 rnHsTyOp env overall_ty (L loc op)
-  = do { ops_ok <- xoptM LangExt.TypeOperators
-       ; op' <- rnTyVar env op
-       ; unless (ops_ok || op' `hasKey` eqTyConKey) $
-           addErr $ TcRnUnknownMessage $ mkPlainError noHints (opTyErr op overall_ty)
-       ; let l_op' = L loc op'
-       ; return (l_op', unitFV op') }
+  = do { op' <- rnTyVar env op
+       ; unlessXOptM LangExt.TypeOperators $
+           if (op' `hasKey` eqTyConKey) -- See [eqTyCon (~) compatibility fallback] in GHC.Rename.Env
+           then addDiagnostic TcRnTypeEqualityRequiresOperators
+           else addErr $ TcRnIllegalTypeOperator overall_ty op
+       ; return (L loc op', unitFV op') }
 
 --------------
 checkWildCard :: RnTyKiEnv
@@ -1660,11 +1659,6 @@ warnUnusedForAll doc (L loc tv) used_names
                    , inHsDocContext doc ]
       addDiagnosticAt (locA loc) msg
 
-opTyErr :: Outputable a => RdrName -> a -> SDoc
-opTyErr op overall_ty
-  = hang (text "Illegal operator" <+> quotes (ppr op) <+> text "in type" <+> quotes (ppr overall_ty))
-         2 (text "Use TypeOperators to allow operators in types")
-
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index ad78cfaf76..2c9b013e17 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -651,6 +651,23 @@ instance Diagnostic TcRnMessage where
             fsep [ text "The use of" <+> quotes (ppr rdr_name)
                                      <+> text "as an identifier",
                    text "will become an error in a future GHC release." ]
+    TcRnTypeEqualityOutOfScope
+      -> mkDecorated
+           [ text "The" <+> quotes (text "~") <+> text "operator is out of scope." $$
+             text "Assuming it to stand for an equality constraint."
+           , text "NB:" <+> (quotes (text "~") <+> text "used to be built-in syntax but now is a regular type operator" $$
+                             text "exported from Data.Type.Equality and Prelude.") $$
+             text "If you are using a custom Prelude, consider re-exporting it."
+           , text "This will become an error in a future GHC release." ]
+    TcRnTypeEqualityRequiresOperators
+      -> mkSimpleDecorated $
+            fsep [ text "The use of" <+> quotes (text "~")
+                                     <+> text "without TypeOperators",
+                   text "will become an error in a future GHC release." ]
+    TcRnIllegalTypeOperator overall_ty op
+      -> mkSimpleDecorated $
+           text "Illegal operator" <+> quotes (ppr op) <+>
+           text "in type" <+> quotes (ppr overall_ty)
     TcRnGADTMonoLocalBinds
       -> mkSimpleDecorated $
             fsep [ text "Pattern matching on GADTs without MonoLocalBinds"
@@ -920,6 +937,12 @@ instance Diagnostic TcRnMessage where
       -> ErrorWithoutFlag
     TcRnForallIdentifier {}
       -> WarningWithFlag Opt_WarnForallIdentifier
+    TcRnTypeEqualityOutOfScope
+      -> WarningWithFlag Opt_WarnTypeEqualityOutOfScope
+    TcRnTypeEqualityRequiresOperators
+      -> WarningWithFlag Opt_WarnTypeEqualityRequiresOperators
+    TcRnIllegalTypeOperator {}
+      -> ErrorWithoutFlag
     TcRnGADTMonoLocalBinds {}
       -> WarningWithFlag Opt_WarnGADTMonoLocalBinds
     TcRnIncorrectNameSpace {}
@@ -1152,6 +1175,12 @@ instance Diagnostic TcRnMessage where
       -> noHints
     TcRnForallIdentifier {}
       -> [SuggestRenameForall]
+    TcRnTypeEqualityOutOfScope
+      -> noHints
+    TcRnTypeEqualityRequiresOperators
+      -> [suggestExtension LangExt.TypeOperators]
+    TcRnIllegalTypeOperator {}
+      -> [suggestExtension LangExt.TypeOperators]
     TcRnGADTMonoLocalBinds {}
       -> [suggestAnyExtension [LangExt.GADTs, LangExt.TypeFamilies]]
     TcRnIncorrectNameSpace nm is_th_use
diff --git a/compiler/GHC/Tc/Errors/Types.hs b/compiler/GHC/Tc/Errors/Types.hs
index abf574df19..58e984011a 100644
--- a/compiler/GHC/Tc/Errors/Types.hs
+++ b/compiler/GHC/Tc/Errors/Types.hs
@@ -1581,6 +1581,35 @@ data TcRnMessage where
   -}
   TcRnForallIdentifier :: RdrName -> TcRnMessage
 
+  {-| TcRnTypeEqualityOutOfScope is a warning (controlled by -Wtype-equality-out-of-scope)
+      that occurs when the type equality (a ~ b) is not in scope.
+
+      Test case: T18862b
+  -}
+  TcRnTypeEqualityOutOfScope :: TcRnMessage
+
+  {-| TcRnTypeEqualityRequiresOperators is a warning (controlled by -Wtype-equality-requires-operators)
+      that occurs when the type equality (a ~ b) is used without the TypeOperators extension.
+
+      Example:
+        {-# LANGUAGE NoTypeOperators #-}
+        f :: (a ~ b) => a -> b
+
+      Test case: T18862a
+  -}
+  TcRnTypeEqualityRequiresOperators :: TcRnMessage
+
+  {-| TcRnIllegalTypeOperator is an error that occurs when a type operator
+      is used without the TypeOperators extension.
+
+      Example:
+        {-# LANGUAGE NoTypeOperators #-}
+        f :: Vec a n -> Vec a m -> Vec a (n + m)
+
+      Test case: T12811
+  -}
+  TcRnIllegalTypeOperator :: !SDoc -> !RdrName -> TcRnMessage
+
   {-| TcRnGADTMonoLocalBinds is a warning controlled by -Wgadt-mono-local-binds
       that occurs when pattern matching on a GADT when -XMonoLocalBinds is off.
 
diff --git a/compiler/GHC/Tc/Validity.hs b/compiler/GHC/Tc/Validity.hs
index 1a5f2f6a41..598b07b8c7 100644
--- a/compiler/GHC/Tc/Validity.hs
+++ b/compiler/GHC/Tc/Validity.hs
@@ -1137,7 +1137,9 @@ check_class_pred env dflags ctxt pred cls tys
                          -- but here we want to treat them as equalities
   = -- Equational constraints are valid in all contexts, and
     -- we do not need to check e.g. for FlexibleContexts here, so just do nothing
-    return ()
+    -- We used to require TypeFamilies/GADTs for equality constraints,
+    -- but not anymore (GHC Proposal #371)
+   return ()
 
   | isIPClass cls
   = do { check_arity
diff --git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index 3aff044b78..5a3794ee37 100644
--- a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ -154,6 +154,7 @@ Library
        ,BangPatterns
        ,ScopedTypeVariables
        ,MonoLocalBinds
+       ,TypeOperators
 
     Exposed-Modules:
         GHC
diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst
index a2cced7294..caac4e9362 100644
--- a/docs/users_guide/9.4.1-notes.rst
+++ b/docs/users_guide/9.4.1-notes.rst
@@ -22,9 +22,24 @@ Language
   If you want to retain the old behavior, add a (backward-compatible) type signature,
   explicitly requesting this unusual quantification.
 
-- GHC no longer checks for ``-XGADTs`` or ``-XTypeFamilies`` in order to use
-  an equality constraint in a type. This is part of accepted proposal
-  `#371 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`_.
+- GHC Proposal `#371 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`_ has been implemented. This means:
+
+  * The use of equality constraints no longer requires ``-XGADTs`` or ``-XTypeFamilies``.
+
+  * The use of equality constraint syntax ``a ~ b`` requires ``-XTypeOperators``,
+    otherwise results in a warning (:ghc-flag:`-Wtype-equality-requires-operators`).
+
+  * ``(~)`` is now a legal name for a user-defined type operator:
+    ::
+
+      class a ~ b where
+        ...
+
+    This used to be rejected with "Illegal binding of built-in syntax".
+
+  * The built-in type equality is now exported from ``Data.Type.Equality`` and
+    re-exported from ``Prelude``. When ``(~)`` is not in scope, its use results
+    in a warning (:ghc-flag:`-Wtype-equality-out-of-scope`).
 
 - There were previously cases around functional dependencies and injective
   type families where the result of type inference would depend on the order
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 5b0093b650..5b2c85099b 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -65,6 +65,7 @@ To reverse ``-Werror``, which makes all warnings into errors, use ``-Wwarn``.
         * :ghc-flag:`-Wunicode-bidirectional-format-characters`
         * :ghc-flag:`-Wforall-identifier`
         * :ghc-flag:`-Wgadt-mono-local-binds`
+        * :ghc-flag:`-Wtype-equality-requires-operators`
 
 The following flags are simple ways to select standard "packages" of warnings:
 
@@ -161,6 +162,7 @@ The following flags are simple ways to select standard "packages" of warnings:
         * :ghc-flag:`-Wnoncanonical-monoid-instances`
         * :ghc-flag:`-Wstar-is-type`
         * :ghc-flag:`-Wcompat-unqualified-imports`
+        * :ghc-flag:`-Wtype-equality-out-of-scope`
 
 .. ghc-flag:: -Wno-compat
     :shortdesc: Disables all warnings enabled by :ghc-flag:`-Wcompat`.
@@ -2286,6 +2288,48 @@ of ``-W(no-)*``.
     to pattern match on a GADT if neither :extension:`GADTs`
     nor :extension:`TypeFamilies` were enabled.
 
+.. ghc-flag:: -Wtype-equality-out-of-scope
+    :shortdesc: warn when type equality ``a ~ b`` is used despite being out of scope
+    :type: dynamic
+    :reverse: -Wno-type-equality-out-of-scope
+
+    :since: 9.4.1
+
+    In accordance with `GHC Proposal #371
+    <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`__,
+    the type equality syntax ``a ~ b`` is no longer built-in. Instead, ``~`` is
+    a regular type operator that can be imported from ``Data.Type.Equality`` or
+    ``Prelude``.
+
+    To minimize breakage, a compatibility fallback is provided: whenever ``~``
+    is used but is not in scope, the compiler assumes that it stands for a type
+    equality constraint. The warning is triggered by any code that relies on
+    this fallback. It can be addressed by bringing ``~`` into scope explicitly.
+
+    The likely culprit is that you use :extension:`NoImplicitPrelude` and a
+    custom Prelude. In this case, consider updating your custom Prelude to
+    re-export ``~`` from ``Data.Type.Equality``.
+
+    Being part of the :ghc-flag:`-Wcompat` option group, this warning is off by
+    default, but will be switched on in a future GHC release.
+
+.. ghc-flag:: -Wtype-equality-requires-operators
+    :shortdesc: warn when type equality ``a ~ b`` is used despite being out of scope
+    :type: dynamic
+    :reverse: -Wno-type-equality-requires-operators
+
+    :since: 9.4.1
+
+    In accordance with `GHC Proposal #371
+    <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`__,
+    the type equality syntax ``a ~ b`` is no longer built-in. Instead, ``~`` is
+    a regular type operator that requires the :extension:`TypeOperators` extension.
+
+    To minimize breakage, ``~`` specifically (unlike other type operators) can
+    be used even when :extension:`TypeOperators` is disabled. The warning is
+    triggered whenever this happens, and can be addressed by enabling the
+    extension.
+
 If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice.
 It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
 sanity, not yours.)
diff --git a/libraries/base/Data/String.hs b/libraries/base/Data/String.hs
index 02a720f8e9..bd5d3d0edd 100644
--- a/libraries/base/Data/String.hs
+++ b/libraries/base/Data/String.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 
 -----------------------------------------------------------------------------
 -- |
diff --git a/libraries/base/Data/Type/Equality.hs b/libraries/base/Data/Type/Equality.hs
index 56cdce77f0..3b11add5f0 100644
--- a/libraries/base/Data/Type/Equality.hs
+++ b/libraries/base/Data/Type/Equality.hs
@@ -31,7 +31,9 @@
 
 module Data.Type.Equality (
   -- * The equality types
-  (:~:)(..), type (~~),
+  type (~),
+  type (~~),
+  (:~:)(..),
   (:~~:)(..),
 
   -- * Working with equality
diff --git a/libraries/base/Data/Type/Ord.hs b/libraries/base/Data/Type/Ord.hs
index 7e9ef24c84..4faf106741 100644
--- a/libraries/base/Data/Type/Ord.hs
+++ b/libraries/base/Data/Type/Ord.hs
@@ -38,6 +38,7 @@ import GHC.Show(Show(..))
 import GHC.TypeError
 import GHC.TypeLits.Internal
 import GHC.TypeNats.Internal
+import GHC.Types (type (~))
 import Data.Bool
 import Data.Char(Char)
 import Data.Eq
diff --git a/libraries/base/Prelude.hs b/libraries/base/Prelude.hs
index 66ea603e72..2e21888abc 100644
--- a/libraries/base/Prelude.hs
+++ b/libraries/base/Prelude.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE ExplicitNamespaces #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -159,6 +160,8 @@ module Prelude (
     -- ** Exception handling in the I\/O monad
     IOError, ioError, userError,
 
+    -- ** The equality types
+    type (~)
   ) where
 
 import Control.Monad
diff --git a/libraries/base/Text/Printf.hs b/libraries/base/Text/Printf.hs
index 4fa9fd7720..397f916265 100644
--- a/libraries/base/Text/Printf.hs
+++ b/libraries/base/Text/Printf.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Safe #-}
-{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeOperators #-}
 {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
 
 -----------------------------------------------------------------------------
diff --git a/libraries/ghc-heap/GHC/Exts/Heap.hs b/libraries/ghc-heap/GHC/Exts/Heap.hs
index a587739813..618419ee1f 100644
--- a/libraries/ghc-heap/GHC/Exts/Heap.hs
+++ b/libraries/ghc-heap/GHC/Exts/Heap.hs
@@ -1,7 +1,7 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
-
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeFamilies #-}
diff --git a/libraries/ghc-prim/GHC/Types.hs b/libraries/ghc-prim/GHC/Types.hs
index 145b5ff179..4c6a3c6a17 100644
--- a/libraries/ghc-prim/GHC/Types.hs
+++ b/libraries/ghc-prim/GHC/Types.hs
@@ -37,7 +37,7 @@ module GHC.Types (
         Any,
 
         -- * Type equality
-        type (~~), Coercible,
+        type (~), type (~~), Coercible,
 
         -- * Representation polymorphism
         TYPE, Levity(..), RuntimeRep(..),
@@ -254,7 +254,6 @@ newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
 *                                                                      *
                     (~) and Coercible
 
-   NB: (~) is built-in syntax, and hence not explicitly exported
 *                                                                      *
 ********************************************************************* -}
 
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 5823cfafa8..3d3f46d2c4 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -2,7 +2,7 @@
              DeriveGeneric, FlexibleInstances, DefaultSignatures,
              RankNTypes, RoleAnnotations, ScopedTypeVariables,
              MagicHash, KindSignatures, PolyKinds, TypeApplications, DataKinds,
-             GADTs, UnboxedTuples, UnboxedSums, TypeInType,
+             GADTs, UnboxedTuples, UnboxedSums, TypeInType, TypeOperators,
              Trustworthy, DeriveFunctor #-}
 
 {-# OPTIONS_GHC -fno-warn-inline-rule-shadowing #-}
diff --git a/testsuite/tests/backpack/should_fail/bkpfail23.bkp b/testsuite/tests/backpack/should_fail/bkpfail23.bkp
index 831ce42c72..55e533390c 100644
--- a/testsuite/tests/backpack/should_fail/bkpfail23.bkp
+++ b/testsuite/tests/backpack/should_fail/bkpfail23.bkp
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE GADTs, RoleAnnotations #-}
+{-# LANGUAGE GADTs, TypeOperators, RoleAnnotations #-}
 unit p where
     signature H where
         type role F phantom
diff --git a/testsuite/tests/deriving/should_compile/T14933.hs b/testsuite/tests/deriving/should_compile/T14933.hs
index 5ab808365f..854f5f717a 100644
--- a/testsuite/tests/deriving/should_compile/T14933.hs
+++ b/testsuite/tests/deriving/should_compile/T14933.hs
@@ -2,9 +2,12 @@
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE DeriveAnyClass    #-}
 {-# LANGUAGE TypeFamilies      #-}
+{-# LANGUAGE TypeOperators     #-}
+
 module T14933 where
 
 import Data.Kind (Type)
+import Data.Type.Equality (type (~))
 
 class Wrapped s where
   type Unwrapped s :: Type
diff --git a/testsuite/tests/ghci/scripts/T12447.script b/testsuite/tests/ghci/scripts/T12447.script
index 826dec0b63..3a30c65905 100644
--- a/testsuite/tests/ghci/scripts/T12447.script
+++ b/testsuite/tests/ghci/scripts/T12447.script
@@ -1,5 +1,5 @@
 :set -XHaskell2010
-:set -XRankNTypes -XConstraintKinds -XTypeApplications
+:set -XRankNTypes -XConstraintKinds -XTypeApplications -XTypeOperators
 
 import Data.Typeable
 
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout b/testsuite/tests/ghci/scripts/ghci024.stdout
index 20750f5817..db6a7b955e 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout
@@ -16,6 +16,7 @@ warning settings:
   -Wsemigroup
   -Wstar-is-type
   -Wcompat-unqualified-imports
+  -Wtype-equality-out-of-scope
 ~~~~~~~~~~ Testing :set -a
 options currently set: none.
 base language is: GHC2021
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32 b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
index 1c03e5fab8..a693b73aaf 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout-mingw32
@@ -15,6 +15,7 @@ warning settings:
   -Wsemigroup
   -Wstar-is-type
   -Wcompat-unqualified-imports
+  -Wtype-equality-out-of-scope
 ~~~~~~~~~~ Testing :set -a
 options currently set: none.
 base language is: GHC2021
diff --git a/testsuite/tests/indexed-types/should_compile/InstEqContext2.hs b/testsuite/tests/indexed-types/should_compile/InstEqContext2.hs
index ea356a44ad..f693837354 100644
--- a/testsuite/tests/indexed-types/should_compile/InstEqContext2.hs
+++ b/testsuite/tests/indexed-types/should_compile/InstEqContext2.hs
@@ -1,6 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
-{-# LANGUAGE TypeFamilies, EmptyDataDecls #-}
+{-# LANGUAGE TypeFamilies, TypeOperators, EmptyDataDecls #-}
 
 module InstEqContext2  where 
 
diff --git a/testsuite/tests/indexed-types/should_compile/T11361.hs b/testsuite/tests/indexed-types/should_compile/T11361.hs
index 7534734790..d2dc75fe8f 100644
--- a/testsuite/tests/indexed-types/should_compile/T11361.hs
+++ b/testsuite/tests/indexed-types/should_compile/T11361.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
   -- this is needed because |FamHelper a x| /< |Fam a x|
   -- This file compiled with -dunique-increment=-1 made GHC crash before
diff --git a/testsuite/tests/indexed-types/should_compile/T15122.hs b/testsuite/tests/indexed-types/should_compile/T15122.hs
index 49a3de642d..1730f3a826 100644
--- a/testsuite/tests/indexed-types/should_compile/T15122.hs
+++ b/testsuite/tests/indexed-types/should_compile/T15122.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE TypeInType #-}
 module T15122 where
 
diff --git a/testsuite/tests/indexed-types/should_compile/T15352.hs b/testsuite/tests/indexed-types/should_compile/T15352.hs
index 25c4640809..0bb4a2bc63 100644
--- a/testsuite/tests/indexed-types/should_compile/T15352.hs
+++ b/testsuite/tests/indexed-types/should_compile/T15352.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE TypeInType #-} -- or PolyKinds
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 
 module T15352 where
 
diff --git a/testsuite/tests/parser/should_fail/T12811.stderr b/testsuite/tests/parser/should_fail/T12811.stderr
index 3dbf6bc342..ec4c79bef9 100644
--- a/testsuite/tests/parser/should_fail/T12811.stderr
+++ b/testsuite/tests/parser/should_fail/T12811.stderr
@@ -1,6 +1,6 @@
 
 T12811.hs:5:15: error:
     Illegal operator ‘.’ in type ‘foral a . a’
-      Use TypeOperators to allow operators in types
+    Suggested fix: Perhaps you intended to use TypeOperators
 
 T12811.hs:5:15: error: Not in scope: type constructor or class ‘.’
diff --git a/testsuite/tests/parser/should_fail/T15209.stderr b/testsuite/tests/parser/should_fail/T15209.stderr
index 9d1e151cf2..fe1666c37e 100644
--- a/testsuite/tests/parser/should_fail/T15209.stderr
+++ b/testsuite/tests/parser/should_fail/T15209.stderr
@@ -1,2 +1,4 @@
 
-T15209.hs:6:10: error: Not in scope: type constructor or class ‘~#’
+T15209.hs:6:10: error:
+    Not in scope: type constructor or class ‘~#’
+    Suggested fix: Perhaps use ‘~’ (imported from Prelude)
diff --git a/testsuite/tests/partial-sigs/should_fail/T14584.hs b/testsuite/tests/partial-sigs/should_fail/T14584.hs
index 1615b26205..077c80c490 100644
--- a/testsuite/tests/partial-sigs/should_fail/T14584.hs
+++ b/testsuite/tests/partial-sigs/should_fail/T14584.hs
@@ -25,6 +25,7 @@ module T14584 where
 
 import Data.Monoid
 import Data.Kind
+import Data.Type.Equality
 
 data family Sing (a::k)
 
diff --git a/testsuite/tests/partial-sigs/should_fail/T14584.stderr b/testsuite/tests/partial-sigs/should_fail/T14584.stderr
index 7a47f25967..2cc457e635 100644
--- a/testsuite/tests/partial-sigs/should_fail/T14584.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T14584.stderr
@@ -1,25 +1,25 @@
 
-T14584.hs:56:50: warning: [-Wdeferred-type-errors (in -Wdefault)]
+T14584.hs:57:50: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • Could not deduce (m1 ~ *)
       from the context: (Action act, Monoid a, Good m1)
-        bound by the instance declaration at T14584.hs:54:10-89
+        bound by the instance declaration at T14584.hs:55:10-89
       ‘m1’ is a rigid type variable bound by
         the instance declaration
-        at T14584.hs:54:10-89
+        at T14584.hs:55:10-89
     • In the type ‘a’
       In the second argument of ‘fromSing’, namely
         ‘(sing @m @a :: Sing _)’
       In the fourth argument of ‘act’, namely
         ‘(fromSing @m (sing @m @a :: Sing _))’
 
-T14584.hs:56:60: warning: [-Wpartial-type-signatures (in -Wdefault)]
+T14584.hs:57:60: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘a0 :: m’
       Where: ‘a0’ is an ambiguous type variable
              ‘m’ is a rigid type variable bound by
                the instance declaration
-               at T14584.hs:54:10-89
+               at T14584.hs:55:10-89
     • In the first argument of ‘Sing’, namely ‘_’
       In the type ‘Sing _’
       In an expression type signature: Sing _
     • Relevant bindings include
-        monHom :: a -> a (bound at T14584.hs:56:3)
+        monHom :: a -> a (bound at T14584.hs:57:3)
diff --git a/testsuite/tests/perf/compiler/T3064.hs b/testsuite/tests/perf/compiler/T3064.hs
index af3dc2cde7..b0eba0bad2 100644
--- a/testsuite/tests/perf/compiler/T3064.hs
+++ b/testsuite/tests/perf/compiler/T3064.hs
@@ -2,6 +2,7 @@
 {-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
 {-# LANGUAGE Rank2Types, TypeSynonymInstances, FlexibleInstances #-}
 {-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE TypeOperators #-}
 
 module T3064 where
 import Control.Applicative
diff --git a/testsuite/tests/polykinds/T11523.hs b/testsuite/tests/polykinds/T11523.hs
index 97551b2208..679d1d13d5 100644
--- a/testsuite/tests/polykinds/T11523.hs
+++ b/testsuite/tests/polykinds/T11523.hs
@@ -19,7 +19,8 @@
 
 module T11523 where
 
-import GHC.Types (Constraint, Type)
+import Data.Kind (Constraint, Type)
+import Data.Type.Equality (type (~))
 import qualified Prelude
 
 type Cat i = i -> i -> Type
diff --git a/testsuite/tests/polykinds/T12055.hs b/testsuite/tests/polykinds/T12055.hs
index de3ad455ea..72c208d77d 100644
--- a/testsuite/tests/polykinds/T12055.hs
+++ b/testsuite/tests/polykinds/T12055.hs
@@ -17,8 +17,8 @@
 
 module T12055 where
 
-import GHC.Base ( Constraint, Type )
-import GHC.Exts ( type (~~) )
+import Data.Kind (Constraint, Type)
+import Data.Type.Equality (type (~), type (~~))
 
 type Cat k = k -> k -> Type
 
diff --git a/testsuite/tests/quantified-constraints/T15359.hs b/testsuite/tests/quantified-constraints/T15359.hs
index 2a58d89c05..ca84c0e8be 100644
--- a/testsuite/tests/quantified-constraints/T15359.hs
+++ b/testsuite/tests/quantified-constraints/T15359.hs
@@ -1,6 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE MultiParamTypeClasses, GADTs, RankNTypes,
-             ConstraintKinds, QuantifiedConstraints #-}
+             ConstraintKinds, QuantifiedConstraints, TypeOperators #-}
 
 module T15359 where
 
diff --git a/testsuite/tests/quantified-constraints/T15359a.hs b/testsuite/tests/quantified-constraints/T15359a.hs
index a5b16acf75..277793b81c 100644
--- a/testsuite/tests/quantified-constraints/T15359a.hs
+++ b/testsuite/tests/quantified-constraints/T15359a.hs
@@ -1,6 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE MultiParamTypeClasses, GADTs, RankNTypes,
-             ConstraintKinds, QuantifiedConstraints,
+             ConstraintKinds, QuantifiedConstraints, TypeOperators,
              UndecidableInstances #-}
 
 module T15359a where
diff --git a/testsuite/tests/rebindable/T19167.hs b/testsuite/tests/rebindable/T19167.hs
index 6f7ebff33d..4e98e815ea 100644
--- a/testsuite/tests/rebindable/T19167.hs
+++ b/testsuite/tests/rebindable/T19167.hs
@@ -6,6 +6,7 @@ module Bug where
 import qualified Prelude as P
 import qualified GHC.Exts as P
 import Data.List.NonEmpty ( NonEmpty )
+import Data.Type.Equality ( type (~) )
 
 fromInteger :: P.Integer -> forall a. P.Num a => a
 fromInteger n = P.fromInteger n
diff --git a/testsuite/tests/rename/should_fail/T15214.hs b/testsuite/tests/rename/should_compile/T15214.hs
index 55f15593be..55f15593be 100644
--- a/testsuite/tests/rename/should_fail/T15214.hs
+++ b/testsuite/tests/rename/should_compile/T15214.hs
diff --git a/testsuite/tests/rename/should_compile/T18862.hs b/testsuite/tests/rename/should_compile/T18862.hs
new file mode 100644
index 0000000000..7356a27ad8
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T18862.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE PolyKinds, DataKinds, TypeOperators, TypeFamilies #-}
+
+module T18862 where
+
+import Prelude (Bool)
+import Data.Kind (Constraint)
+import qualified Data.Type.Equality as E
+
+type family (a :: k) ~ (b :: k) :: result_kind
+
+type instance a ~ b = (a E.~ b :: Constraint)
+type instance a ~ b = (a E.== b :: Bool)
diff --git a/testsuite/tests/rename/should_compile/all.T b/testsuite/tests/rename/should_compile/all.T
index 536c5b9013..3a6fdceac3 100644
--- a/testsuite/tests/rename/should_compile/all.T
+++ b/testsuite/tests/rename/should_compile/all.T
@@ -161,6 +161,7 @@ test('T14881', [], multimod_compile, ['T14881', '-W'])
 test('T14487', [], multimod_compile, ['T14487', '-v0'])
 test('T14747', [], multimod_compile, ['T14747', '-v0'])
 test('T15149', [], multimod_compile, ['T15149', '-v0'])
+test('T15214', normal, compile, [''])
 test('T13064', normal, compile, [''])
 test('T15994', [], makefile_test, ['T15994'])
 test('T15798a', normal, compile, [''])
@@ -184,3 +185,4 @@ test('T20609a', normal, compile, [''])
 test('T20609b', normal, compile, [''])
 test('T20609c', normal, compile, [''])
 test('T20609d', normal, compile, [''])
+test('T18862', normal, compile, [''])
diff --git a/testsuite/tests/rename/should_fail/T15214.stderr b/testsuite/tests/rename/should_fail/T15214.stderr
deleted file mode 100644
index 399438adb5..0000000000
--- a/testsuite/tests/rename/should_fail/T15214.stderr
+++ /dev/null
@@ -1,2 +0,0 @@
-
-T15214.hs:4:1: error: Illegal binding of built-in syntax: ~
diff --git a/testsuite/tests/rename/should_fail/all.T b/testsuite/tests/rename/should_fail/all.T
index 4ce00de399..5e62e28847 100644
--- a/testsuite/tests/rename/should_fail/all.T
+++ b/testsuite/tests/rename/should_fail/all.T
@@ -137,7 +137,6 @@ test('T14307', normal, compile_fail, [''])
 test('T14591', normal, compile_fail, [''])
 test('T14907a', normal, compile_fail, [''])
 test('T14907b', normal, compile_fail, [''])
-test('T15214', normal, compile_fail, [''])
 test('T15539', normal, compile_fail, [''])
 test('T15487', normal, multimod_compile_fail, ['T15487','-v0'])
 test('T15659', normal, compile_fail, [''])
diff --git a/testsuite/tests/roles/should_compile/Roles3.hs b/testsuite/tests/roles/should_compile/Roles3.hs
index b71d9cf2ec..b79129162e 100644
--- a/testsuite/tests/roles/should_compile/Roles3.hs
+++ b/testsuite/tests/roles/should_compile/Roles3.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}  -- meth3, meth4 are ambiguous
 
 module Roles3 where
@@ -20,4 +21,4 @@ class C4 a b where
   meth4 :: a -> F4 b -> F4 b
 
 type Syn1 a = F4 a
-type Syn2 a = [a]
\ No newline at end of file
+type Syn2 a = [a]
diff --git a/testsuite/tests/tcplugins/TcPlugin_Rewrite.hs b/testsuite/tests/tcplugins/TcPlugin_Rewrite.hs
index 6df19b7b62..5cacd28618 100644
--- a/testsuite/tests/tcplugins/TcPlugin_Rewrite.hs
+++ b/testsuite/tests/tcplugins/TcPlugin_Rewrite.hs
@@ -2,13 +2,16 @@
 {-# OPTIONS_GHC -fplugin RewritePlugin #-}
 
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE DataKinds, ScopedTypeVariables, TypeFamilies #-}
+{-# LANGUAGE DataKinds, ScopedTypeVariables, TypeFamilies, TypeOperators #-}
 
 module TcPlugin_Rewrite where
 
 import Data.Kind
   ( Type )
 
+import Data.Type.Equality
+  ( type (~) )
+
 import Definitions
   ( Add, Nat(..) )
 
diff --git a/testsuite/tests/typecheck/should_compile/LocalGivenEqs.hs b/testsuite/tests/typecheck/should_compile/LocalGivenEqs.hs
index fd37ede0db..43063d20aa 100644
--- a/testsuite/tests/typecheck/should_compile/LocalGivenEqs.hs
+++ b/testsuite/tests/typecheck/should_compile/LocalGivenEqs.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE RankNTypes, TypeFamilies, FlexibleInstances #-}
+{-# LANGUAGE RankNTypes, TypeFamilies, TypeOperators, FlexibleInstances #-}
 {-# OPTIONS_GHC -Wno-missing-methods -Wno-unused-matches #-}
 
 module LocalGivenEqs where
diff --git a/testsuite/tests/typecheck/should_compile/T11524.hs b/testsuite/tests/typecheck/should_compile/T11524.hs
index 0856afe50b..a8c40c88c6 100644
--- a/testsuite/tests/typecheck/should_compile/T11524.hs
+++ b/testsuite/tests/typecheck/should_compile/T11524.hs
@@ -3,6 +3,7 @@
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE TypeOperators #-}
 
 module T11524 where
 
diff --git a/testsuite/tests/typecheck/should_compile/T12919.hs b/testsuite/tests/typecheck/should_compile/T12919.hs
index 29eb9dc469..825b7f65c1 100644
--- a/testsuite/tests/typecheck/should_compile/T12919.hs
+++ b/testsuite/tests/typecheck/should_compile/T12919.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, GADTs, ConstraintKinds #-}
+{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, GADTs, ConstraintKinds, TypeOperators #-}
 
 module T12919 where
 
diff --git a/testsuite/tests/typecheck/should_compile/T13651a.hs b/testsuite/tests/typecheck/should_compile/T13651a.hs
index f08407b738..80cda684e1 100644
--- a/testsuite/tests/typecheck/should_compile/T13651a.hs
+++ b/testsuite/tests/typecheck/should_compile/T13651a.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE TypeFamilies, TypeFamilyDependencies #-}
+{-# LANGUAGE TypeFamilies, TypeFamilyDependencies, TypeOperators #-}
 module T13651 where
 
 type family F r s = f | f -> r s
diff --git a/testsuite/tests/typecheck/should_compile/T17202.hs b/testsuite/tests/typecheck/should_compile/T17202.hs
index cda5783ec9..ee639b9cc0 100644
--- a/testsuite/tests/typecheck/should_compile/T17202.hs
+++ b/testsuite/tests/typecheck/should_compile/T17202.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE ConstraintKinds #-}
 
 module T17202 where
diff --git a/testsuite/tests/typecheck/should_compile/T17562b.hs b/testsuite/tests/typecheck/should_compile/T17562b.hs
index b73e6b1bc7..44eb03ce97 100644
--- a/testsuite/tests/typecheck/should_compile/T17562b.hs
+++ b/testsuite/tests/typecheck/should_compile/T17562b.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE QuantifiedConstraints, MultiParamTypeClasses, TypeFamilies #-}
+{-# LANGUAGE QuantifiedConstraints, MultiParamTypeClasses, TypeFamilies, TypeOperators #-}
 -- NB: No PolyKinds
 
 module T17562b where
diff --git a/testsuite/tests/typecheck/should_compile/T17567StupidThetaB.hs b/testsuite/tests/typecheck/should_compile/T17567StupidThetaB.hs
index d555511004..1d397f9861 100644
--- a/testsuite/tests/typecheck/should_compile/T17567StupidThetaB.hs
+++ b/testsuite/tests/typecheck/should_compile/T17567StupidThetaB.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE Haskell2010 #-}
-{-# LANGUAGE QuantifiedConstraints, DatatypeContexts, TypeFamilies #-}
+{-# LANGUAGE QuantifiedConstraints, DatatypeContexts, TypeFamilies, TypeOperators #-}
 -- NB: -XNoPolyKinds, to get defaulting.
 
 module T17567StupidThetaB where
diff --git a/testsuite/tests/typecheck/should_compile/T21010A.hs b/testsuite/tests/typecheck/should_compile/T21010A.hs
index 234e1c72d7..5e8ef6fe96 100644
--- a/testsuite/tests/typecheck/should_compile/T21010A.hs
+++ b/testsuite/tests/typecheck/should_compile/T21010A.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 
 module T21010A ( WrapMono, Constrained(..), withMonoCoercible ) where
 import T21010B ( WrapMono(..), withMonoCoercible )
diff --git a/testsuite/tests/typecheck/should_compile/tc250.hs b/testsuite/tests/typecheck/should_compile/tc250.hs
index 445d702b69..1076fc5f49 100644
--- a/testsuite/tests/typecheck/should_compile/tc250.hs
+++ b/testsuite/tests/typecheck/should_compile/tc250.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 module ShouldCompile where
 
 import Data.Kind (Type)
diff --git a/testsuite/tests/typecheck/should_compile/tc251.hs b/testsuite/tests/typecheck/should_compile/tc251.hs
index e9315263af..1c69c1d9ac 100644
--- a/testsuite/tests/typecheck/should_compile/tc251.hs
+++ b/testsuite/tests/typecheck/should_compile/tc251.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 module ShouldCompile where
 
 import Tc251_Help
diff --git a/testsuite/tests/typecheck/should_compile/tc252.hs b/testsuite/tests/typecheck/should_compile/tc252.hs
index 1ad9c7c514..6ff23e75e4 100644
--- a/testsuite/tests/typecheck/should_compile/tc252.hs
+++ b/testsuite/tests/typecheck/should_compile/tc252.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 module ShouldCompile where
 
 import Data.Kind (Type)
diff --git a/testsuite/tests/typecheck/should_compile/tc253.hs b/testsuite/tests/typecheck/should_compile/tc253.hs
index 7553b756a8..5c032daa01 100644
--- a/testsuite/tests/typecheck/should_compile/tc253.hs
+++ b/testsuite/tests/typecheck/should_compile/tc253.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Haskell2010 #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
   -- this is needed because |FamHelper a x| /< |Fam a x|
 module ShouldCompile where
diff --git a/testsuite/tests/warnings/should_compile/T18862a.hs b/testsuite/tests/warnings/should_compile/T18862a.hs
new file mode 100644
index 0000000000..532cd83e83
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18862a.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE NoTypeOperators #-}
+
+module T18862a where
+
+f :: (a ~ b) => a -> b
+f = id
diff --git a/testsuite/tests/warnings/should_compile/T18862a.stderr b/testsuite/tests/warnings/should_compile/T18862a.stderr
new file mode 100644
index 0000000000..e2a6fe3f3c
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18862a.stderr
@@ -0,0 +1,5 @@
+
+T18862a.hs:5:9: warning: [-Wtype-equality-requires-operators (in -Wdefault)]
+    The use of ‘~’ without TypeOperators
+    will become an error in a future GHC release.
+    Suggested fix: Perhaps you intended to use TypeOperators
diff --git a/testsuite/tests/warnings/should_compile/T18862b.hs b/testsuite/tests/warnings/should_compile/T18862b.hs
new file mode 100644
index 0000000000..7259547b7c
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18862b.hs
@@ -0,0 +1,8 @@
+{-# OPTIONS -Wcompat -Wno-error=type-equality-out-of-scope #-}
+
+module T18862b where
+
+import Prelude (id)
+
+f :: (a ~ b) => a -> b
+f = id
diff --git a/testsuite/tests/warnings/should_compile/T18862b.stderr b/testsuite/tests/warnings/should_compile/T18862b.stderr
new file mode 100644
index 0000000000..ac959be7d8
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T18862b.stderr
@@ -0,0 +1,8 @@
+
+T18862b.hs:7:9: warning: [-Wtype-equality-out-of-scope (in -Wcompat)]
+    • The ‘~’ operator is out of scope.
+      Assuming it to stand for an equality constraint.
+    • NB: ‘~’ used to be built-in syntax but now is a regular type operator
+          exported from Data.Type.Equality and Prelude.
+      If you are using a custom Prelude, consider re-exporting it.
+    • This will become an error in a future GHC release.
diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T
index 2934db7ad4..3c514245fe 100644
--- a/testsuite/tests/warnings/should_compile/all.T
+++ b/testsuite/tests/warnings/should_compile/all.T
@@ -47,3 +47,5 @@ test('T19564d', normal, compile, [''])
 test('T19296', normal, compile, ['-fdiagnostics-show-caret -Wredundant-constraints -dsuppress-uniques'])
 test('DodgyExports01', normal, compile, ['-Wdodgy-exports'])
 test('DerivingTypeable', normal, compile, ['-Wderiving-typeable'])
+test('T18862a', normal, compile, [''])
+test('T18862b', normal, compile, [''])
diff --git a/utils/check-exact/ExactPrint.hs b/utils/check-exact/ExactPrint.hs
index 3fb283e1ff..2c96ff49ea 100644
--- a/utils/check-exact/ExactPrint.hs
+++ b/utils/check-exact/ExactPrint.hs
@@ -11,6 +11,7 @@
 {-# LANGUAGE ViewPatterns         #-}
 {-# LANGUAGE ScopedTypeVariables  #-}
 {-# LANGUAGE TypeApplications     #-}
+{-# LANGUAGE TypeOperators        #-}
 {-# LANGUAGE UndecidableInstances  #-} -- For the (StmtLR GhcPs GhcPs (LocatedA (body GhcPs))) ExactPrint instance
 
 module ExactPrint
diff --git a/utils/haddock b/utils/haddock
-Subproject e057bfc880d98fe872e3ee9291d2ee1cd3ceecc
+Subproject b02188ab1cc46dd82395a22b04f890cf15f3fea