Add documentation for GHC-06202 (TcRnMisplacedInstSig)

Author: dyniec

message-index/messages/GHC-06202/example1/after/Example1.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE Haskell2010, InstanceSigs #-}
+module Example1 where
+
+data A = A | B
+instance Eq A where
+    (==) :: A -> A -> Bool
+    A == A = True
+    B == B = True
+    _ == _ = False

message-index/messages/GHC-06202/example1/before/Example1.hs

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+{-# LANGUAGE Haskell2010 #-}
+module Example1 where
+
+data A = A | B
+instance Eq A where
+    (==) :: A -> A -> Bool
+    A == A = True
+    B == B = True
+    _ == _ = False

message-index/messages/GHC-06202/example1/index.md

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+---
+title: Illegal type signature in instance declaration
+---
+
+## Error message
+
+Type signatures are only allowed to be in instance declarations when the `InstanceSigs` language extension is enabled.
+
+```
+Example1.hs:6:13: error: [GHC-06202]
+    • Illegal type signature in instance declaration:
+        (==) :: A -> A -> Bool
+    • In the instance declaration for ‘Eq A’
+    Suggested fix: Perhaps you intended to use InstanceSigs
+  |
+6 |     (==) :: A -> A -> Bool
+```

message-index/messages/GHC-06202/index.md

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+---
+title: Illegal type signature in instance declaration
+summary: Method in class instance has type signature, but extension InstanceSigs is not enabled
+severity: error
+introduced: 9.6.1
+---
+
+This error is emitted when class instance has type signature, but extension [InstanceSigs](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/instances.html#instance-signatures-type-signatures-in-instance-declarations) is not enabled. Note that `InstanceSigs` is part of `GHC2021` and `GHC2024` [editions](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/control.html), so you won't encounter this error in new code unless you explicitely opt-out.