Document GHC-12003

Fixes #231.

message-index/messages/GHC-12003/index.md

@@ -0,0 +1,9 @@
+---
+title: Type equality not in scope
+summary: The type equality operator has not been imported into the current module
+severity: warning
+flag: "-Wcompat"
+introduced: 9.6.1
+---
+
+In prior versions of GHC, the type equality operator `~` was built-in syntax. In more recent versions, it is an ordinary operator that is part of the `Prelude` and of `Data.Type.Equality`. Restricting imports from `Prelude` can result in `~` not being imported. For now, GHC has a compatibility warning to help migrate old code. In a future version of GHC, this warning will be an error.

message-index/messages/GHC-12003/out-of-scope/after/TypeEqNotImported.hs

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+{-# LANGUAGE ExplicitNamespaces #-}
+{-# OPTIONS -Wcompat #-}
+module TypeEqNotImported where
+
+import Prelude (id, type (~))
+
+f :: (a ~ b) => a -> b
+f = id

message-index/messages/GHC-12003/out-of-scope/before/TypeEqNotImported.hs

@@ -0,0 +1,8 @@
+
+{-# OPTIONS -Wcompat #-}
+module TypeEqNotImported where
+
+import Prelude (id)
+
+f :: (a ~ b) => a -> b
+f = id

message-index/messages/GHC-12003/out-of-scope/index.md

@@ -0,0 +1,5 @@
+---
+title: Type equality not imported from Prelude
+---
+
+In this example, the type equality operator was not imported from the `Prelude`. To remove the warning, it must be imported.