Merge pull request #445 from jhrcek/GHC-97044

Document GHC-97044

Author: BinderDavid

message-index/messages/GHC-97044/example1/after/UserSpecifiedInstances.hs

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+module Main where
+
+import Data.Coerce
+import Data.Type.Equality
+import Type.Reflection
+
+newtype Foo = Foo { unFoo :: () }
+
+main :: IO ()
+main = do
+    let foo = Foo ()
+    print (coerce foo:: ())
+    print (typeOf foo)
+    print (Refl :: Foo :~: Foo)

message-index/messages/GHC-97044/example1/before/UserSpecifiedInstances.hs

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+module Main where
+
+import Data.Coerce (Coercible, coerce)
+import Type.Reflection (Typeable(..))
+import Data.Type.Equality
+
+newtype Foo = Foo { unFoo :: () }
+
+instance Coercible Foo ()
+
+instance Typeable Foo
+
+instance Foo ~ Foo
+
+main :: IO ()
+main = do
+    let foo = Foo ()
+    print (coerce foo:: ())
+    print (typeOf foo)
+    print (Refl :: Foo :~: Foo)

message-index/messages/GHC-97044/example1/index.md

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+---
+title: Attempts to create hand-written instances for special type classes
+---
+
+Since these type classes don't allow hand-written instances, you'll get the following errors if you try to implement them.
+To fix the errors, just remove the instance declarations - GHC will provide them for you automatically.
+
+## Error Message
+```
+UserSpecifiedInstances.hs:9:10: error: [GHC-97044]
+    • Class ‘Coercible’ does not support user-specified instances.
+    • In the instance declaration for ‘Coercible Foo ()’
+  |
+9 | instance Coercible Foo ()
+  |          ^^^^^^^^^^^^^^^^
+
+UserSpecifiedInstances.hs:11:10: error: [GHC-97044]
+    • Class ‘Typeable’ does not support user-specified instances.
+    • In the instance declaration for ‘Typeable Foo’
+   |
+11 | instance Typeable Foo
+   |          ^^^^^^^^^^^^
+
+UserSpecifiedInstances.hs:13:10: error: [GHC-97044]
+    • Class ‘~’ does not support user-specified instances.
+    • In the instance declaration for ‘Foo ~ Foo’
+   |
+13 | instance Foo ~ Foo
+   |          ^^^^^^^^^
+```

message-index/messages/GHC-97044/index.md

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+---
+title: User-specified instance is not allowed
+summary: Type class does not allow user-specified instances
+severity: error
+introduced: 9.6.1
+---
+
+There are several special type classes in [base](https://hackage.haskell.org/package/base) which require special treatment from the compiler. For this reason they don't permit user-specified instances.
+
+An attempt to provide an explicit instance declaration in your library / application code, e.g.
+```haskell
+instance Coercible A B where ...
+```
+will lead to this error being reported.
+
+The instances for these type classes are automatically created by GHC on an as-needed basis.
+
+This restriction applies to the following type classes:
+
+- [Coercible](https://hackage.haskell.org/package/base/docs/Data-Coerce.html#t:Coercible) - This class does not have regular instances; instead they are created on-the-fly during type-checking.
+- [Typeable](https://hackage.haskell.org/package/base/docs/Type-Reflection.html#t:Typeable) - Since GHC 7.10, all types automatically have `Typeable` instances derived. This is in contrast to previous releases where Typeable had to be explicitly derived using the DeriveDataTypeable language extension.
+- Type equality classes `~` and `~~` defined in [Data.Type.Equality](https://hackage.haskell.org/package/base/docs/Data-Type-Equality.html)
+- [WithDict](https://hackage.haskell.org/package/base/docs/GHC-Exts.html#t:WithDict) - is used to create dictionaries for classes with a single method. It is used to implement a primitive that we cannot define in Haskell but we can write in Core. Some details can be found in a [Note in GHC library](https://hackage.haskell.org/package/ghc-9.6.1/docs/src/GHC.Tc.Instance.Class.html#line-493)
+- [KnownChar](https://hackage.haskell.org/package/base/docs/GHC-TypeLits.html#t:KnownChar) - This class gives the Char associated with a type-level Char literal. There are instances of the class for every concrete literal: 'a', 'b', 'c' etc. But instances for user-defined types are not permitted.
+- [KnownNat](https://hackage.haskell.org/package/base/docs/GHC-TypeNats.html#t:KnownNat) - This class gives the integer associated with a type-level natural. There are instances of the class for every concrete literal: 0, 1, 2, etc. But instances for user-defined types are not permitted.
+- [KnownSymbol](https://hackage.haskell.org/package/base/docs/GHC-TypeLits.html#t:KnownSymbol) - This class gives the string associated with a type-level symbol. There are instances of the class for every concrete literal: "hello", etc. But instances for user-defined types are not permitted.
+- [Generic](https://hackage.haskell.org/package/base/docs/GHC-Generics.html#t:Generic) - This class is one of the key pieces of `GHC.Generics` and you usually want to derive it using [DeriveGeneric](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/generics.html#extension-DeriveGeneric) extension. Hand-written instances of Generic instances are allowed by default, but disallowed when [SafeHaskell](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/safe_haskell.html) extension is enabled.
+