Basic explanation of 59692

Author: noughtmare

message-index/messages/GHC-59692/example1/after/Example1.hs

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+module Example1 where
+
+data T a
+
+instance Eq a => Eq (T a) where
+  (==) = (==)

message-index/messages/GHC-59692/example1/before/Example1.hs

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+module Example1 where
+
+data T a
+
+instance Eq a => Eq (T a) where
+  (==) = (==)
+
+instance Show a => Eq (T a) where
+  (==) = (==)

message-index/messages/GHC-59692/example1/index.md

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+Even though the instances have different constraints, they are still considered the same.
+
+## Error message
+
+```
+messages/GHC-59692/example1/before/Example1.hs:3:10: error: [GHC-59692]
+    Duplicate instance declarations:
+      instance Eq a => Eq (T a)
+        -- Defined at messages/GHC-59692/example1/before/Example1.hs:3:10
+      instance Show a => Eq (T a)
+        -- Defined at messages/GHC-59692/example1/before/Example1.hs:6:10
+  |
+3 | instance Eq a => Eq (T a) where
+  |          ^^^^^^^^^^^^^^^^
+```

message-index/messages/GHC-59692/index.md

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+---
+title: Duplicate instance declarations
+summary: An instance has been defined twice for the same type.
+introduced: 9.6.1
+severity: error
+---
+
+You can only define a single instance for every type, because it must be possible to automatically resolve class constraints without any ambiguity.
+Note that instances are considered the same even if they have different constraints. Only the arguments of the class are considered when matching instances.
+See the [GHC User Guide](https://downloads.haskell.org/~ghc/9.10.1/docs/users_guide/exts/instances.html) for more information on instance declarations and resolution.