Add docs for GH-09646

Author: TauOmicronMu

message-index/messages/GHC-09646/index.md

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+---
+title: Tuple section in pattern
+summary: There has been an attempt to pattern match on a tuple section.
+severity: error
+introduced: 9.6.1
+---
+
+Pattern matching allows constructors and literals to be used to take apart values, exposing their contents. The syntax of patterns is very similar to that of the expressions that create values, but many constructs that are allowed in expression contexts are not meaningful patterns. 
+
+## Example Text
+
+```
+Tuple section in pattern context
+``` 

message-index/messages/GHC-09646/snd/after/Snd.hs

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+module Snd where
+
+{-# LANGUAGE TupleSections #-}
+
+snd' :: (a, a) -> a 
+snd' (_,x) = x 

message-index/messages/GHC-09646/snd/before/Snd.hs

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+module Snd where
+
+{-# LANGUAGE TupleSections #-}
+
+snd' :: (a, a) -> a 
+snd' (,x) = x 

message-index/messages/GHC-09646/snd/index.md

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+---
+title: Using tuple section in pattern
+---
+
+When pattern matching, tuple sections are not allowed as patterns to be matched against. In the example below, we can fix this by adding the wildcard _ to match on the first element of the pair.  
+
+#  Error Message 
+
+```
+before.hs:4:6: error: [GHC-09646] Tuple section in pattern context
+  |
+4 | snd' (,x) = x 
+  |      ^^^^
+Failed, no modules loaded.
+```