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.

1 files changed, 19 insertions, 8 deletions

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]