Add GHC-02550 documentation

Author: FinleyMcIlwaine

message-index/messages/GHC-02550/example/after/QuantifiedFunction.hs

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+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE TypeFamilies          #-}
+
+module QuantifiedFunction where
+
+import Data.Kind
+
+class C a where
+
+class A a where
+
+f :: (C Int => A Int) => ()
+f = ()

message-index/messages/GHC-02550/example/before/QuantifiedFunction.hs

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+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE TypeFamilies          #-}
+
+module QuantifiedFunction where
+
+import Data.Kind
+
+class C a where
+
+type family A :: k -> Constraint
+
+f :: (C Int => A Int) => ()
+f = ()

message-index/messages/GHC-02550/example/index.md

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+---
+title: Bad quantified constraint in function type
+---
+
+In this example, a quantified constraint occurs in the type of a binding.
+Because the head of the quantified constraint is computed from a type family, we
+get the following error message:
+
+```
+• Quantified predicate must have a class or type variable head:
+    a => A a
+• In the quantified constraint ‘a => A a’
+  In the type signature: f :: (a => A a) => ()
+```
+
+One potential fix for this is to convert the type family into a class.

message-index/messages/GHC-02550/index.md

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+---
+title: Quantified predicate must have a class or type variable head
+summary: The head of a quantified constraint must be either a class or a type variable
+severity: error
+introduced: 9.4.1
+---
+
+Quantified constraints are constraints which have their own constraints. The
+*head* of a quantified constraint is the constraint on the right side of the
+`=>` operator. For example, the head of the quantified constraint `Ord a => Eq
+a` is `Eq a`. This error message states that the head of a quantified constraint
+must not be a constraint resulting from, for example, a type family; it must
+be either a class constraint or a type variable.