Document GHC-72771 "Mismatched default method signature"

Author: VeryMilkyJoe

message-index/messages/GHC-72771/example/after/Lib.hs

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+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE TypeOperators #-}
+
+module Lib where
+
+import GHC.Generics
+
+class Enumerate a where
+  enumerate :: [a]
+  default enumerate :: (Int ~ a) => [a]
+  enumerate = [1 ..]

message-index/messages/GHC-72771/example/before/Lib.hs

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+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE TypeOperators #-}
+
+module Lib where
+
+import GHC.Generics
+
+class Enumerate a where
+  enumerate :: [a]
+  default enumerate :: (Int ~ a) => a
+  enumerate = [1 ..]

message-index/messages/GHC-72771/example/index.md

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+---
+title: Default type signature does not match
+---
+
+## Error Message
+```
+Lib.hs:10:11: error: [GHC-72771]
+    • The default type signature for enumerate: (Int ~ a) => a
+      does not match its corresponding non-default type signature
+    • When checking the class method:
+         enumerate :: forall a. Enumerate a => [a]
+      In the class declaration for ‘Enumerate’
+   |
+10 |   default enumerate :: (Int ~ a) => a
+   |           ^^^^^^^^^
+```
+
+## Explanation
+
+Here, we have defined an `Enumerate` typeclass over some type `a`, its instance `enumerate` simply has the signature `[a]`.
+At the same time, we define a default method for `enumerate`, which is applied in case the `a` is of type `Int`.
+But, this default method has the type signature `a`, which is not equivalent to `[a]`, therefore this error is thrown.
+
+To resolve this error, we need to define the default method signature to match the method signature.
+Therefore, we define the default method signature to produce `[a]` instead of `a`.

message-index/messages/GHC-72771/index.md

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+---
+title: Mismatched default method signature
+summary: The provided default method implementation's type signature does not match the corresponding method type signature.
+severity: error
+introduced: 9.8.1
+---
+
+This error means that there is a type class with some method `m` for which a [default signature](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/exts/default_signatures.html) was defined, but that default signature does not match the type signature of `m`.