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.

3 files changed, 61 insertions, 1 deletions

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