Add explanation for GHC-87139.

Author: kosmikus

message-index/messages/GHC-87139/illegalDerivingStrategy/after/IllegalDerivingStrategy.hs

@@ -0,0 +1,5 @@
+{-# LANGUAGE DerivingStrategies #-}
+module IllegalDerivingStrategy where
+
+newtype Year = MkYear Int
+  deriving newtype Show

message-index/messages/GHC-87139/illegalDerivingStrategy/before/IllegalDerivingStrategy.hs

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+module IllegalDerivingStrategy where
+
+newtype Year = MkYear Int
+  deriving newtype Show

message-index/messages/GHC-87139/illegalDerivingStrategy/index.md

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+---
+title: Illegal deriving strategy on newtype
+---
+
+In this example, there is a deriving clause instructing GHC
+to derive a `Show` instance for the newtype `Year`.
+
+There are (at least) two different ways to derive such a `Show`
+instance. The `stock` strategy is the standard generic way of
+deriving `Show` instances for datatypes. The `newtype` strategy
+is available for `newtype`s and simply uses the existing instance
+of the underlying type, in this case `Int`.
+
+The difference in the current example is that the `stock`-derived
+`show` includes the constructor name, the `newtype`-derived does
+not.
+
+If multiple strategies are available, GHC chooses a default. In
+the case of `deriving` for classes for which `stock`-deriving is
+available, such as `Show`, `stock` is the default. If you want to
+be explicit about this or explicitly deviate from the default,
+you can specify a strategy explicitly.
+
+This, however, requires the `DerivingStrategies` language extension
+to be enabled, and if it is not, you will get an error. The fix in
+this case is to enable the language extension, for example via a
+compiler pragma.

message-index/messages/GHC-87139/index.md

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+---
+title: Illegal deriving strategy
+summary: Use of a deriving strategy without enabling the corresponding language extension
+severity: error
+introduced: 9.8.1
+---
+
+This error occurs if the syntax for specifying a deriving strategy
+is being used without having the corresponding language extension enabled.
+
+The fix is usually to either enable the language extension, or to remove
+the deriving strategy (if it happens to match the default).