Document GHC-46537 Unsupported extension (closes #150)

message-index/messages/GHC-46537/example1/after/UnsupportedExtension.hs

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+{-# LANGUAGE ExistentialQuantification #-}
+
+module UnsupportedExtension where

message-index/messages/GHC-46537/example1/before/UnsupportedExtension.hs

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+{-# LANGUAGE ExistientialQuantification #-}
+-- typo here      ^                
+module UnsupportedExtension where

message-index/messages/GHC-46537/example1/index.md

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+---
+title: Unsupported extension
+---
+
+## Error Message
+```
+Main.hs:1:14: error: [GHC-46537]
+    Unsupported extension: ExistientialQuantification
+    Suggested fix:
+      Perhaps you meant ‘ExistentialQuantification’ or ‘NoExistentialQuantification’
+  |
+1 | {-# LANGUAGE ExistientialQuantification #-}
+  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+```

message-index/messages/GHC-46537/index.md

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+---
+title: Unsupported extension
+summary: GHC failed to recognize name of a language extension
+severity: error
+introduced: 9.6.1
+---
+
+This error is reported when GHC fails to recognize the name of a language extension, provided via `{-# LANGUAGE ... #-}` pragma at the top of a `.hs` file.
+
+There are two likely causes:
+
+1. You've made a typo, in which case ghc will likely also suggest the correct extension name to use.
+2. You're using older version of GHC which doesn't yet support given extension.
+    Here's a couple of things you can try to troubleshoot:
+
+    - Find out what version of GHC you're using:
+
+          ghc --version
+
+    - List all the supported extension that this version of GHC supports. Does the list contain your desired extension?
+
+          ghc --supported-extensions
+
+    - Search for the extension name in the latest version of [GHC User's Guide](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts.html). Most extensions have a `since` annotation which tells you in which version of GHC the extension was introduced.