From 7e45e19191b64d5e408a75188efee20fbfdd70ba Mon Sep 17 00:00:00 2001
From: Finley McIlwaine <finleymcilwaine@gmail.com>
Date: Thu, 6 Jun 2024 17:50:13 +0200
Subject: [PATCH 1/2] Add GHC-02550 documentation

---
 .../example/after/QuantifiedFunction.hs          | 13 +++++++++++++
 .../example/before/QuantifiedFunction.hs         | 13 +++++++++++++
 .../messages/GHC-02550/example/index.md          | 16 ++++++++++++++++
 message-index/messages/GHC-02550/index.md        | 13 +++++++++++++
 4 files changed, 55 insertions(+)
 create mode 100644 message-index/messages/GHC-02550/example/after/QuantifiedFunction.hs
 create mode 100644 message-index/messages/GHC-02550/example/before/QuantifiedFunction.hs
 create mode 100644 message-index/messages/GHC-02550/example/index.md
 create mode 100644 message-index/messages/GHC-02550/index.md

diff --git a/message-index/messages/GHC-02550/example/after/QuantifiedFunction.hs b/message-index/messages/GHC-02550/example/after/QuantifiedFunction.hs
new file mode 100644
index 00000000..2a9f60fd
--- /dev/null
+++ b/message-index/messages/GHC-02550/example/after/QuantifiedFunction.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE TypeFamilies          #-}
+
+module QuantifiedFunction where
+
+import Data.Kind
+
+class C a where
+
+class A a where
+
+f :: (C Int => A Int) => ()
+f = ()
diff --git a/message-index/messages/GHC-02550/example/before/QuantifiedFunction.hs b/message-index/messages/GHC-02550/example/before/QuantifiedFunction.hs
new file mode 100644
index 00000000..9a1b5fc2
--- /dev/null
+++ b/message-index/messages/GHC-02550/example/before/QuantifiedFunction.hs
@@ -0,0 +1,13 @@
+{-# 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 = ()
diff --git a/message-index/messages/GHC-02550/example/index.md b/message-index/messages/GHC-02550/example/index.md
new file mode 100644
index 00000000..cb406ec7
--- /dev/null
+++ b/message-index/messages/GHC-02550/example/index.md
@@ -0,0 +1,16 @@
+---
+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.
diff --git a/message-index/messages/GHC-02550/index.md b/message-index/messages/GHC-02550/index.md
new file mode 100644
index 00000000..aebf1682
--- /dev/null
+++ b/message-index/messages/GHC-02550/index.md
@@ -0,0 +1,13 @@
+---
+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.

From 0ab4c2e5be7e2ca5aabc0c7a6659d5cc3ba3ff70 Mon Sep 17 00:00:00 2001
From: Bodigrim <andrew.lelechenko@gmail.com>
Date: Tue, 25 Mar 2025 22:55:34 +0000
Subject: [PATCH 2/2] Review suggestions for GHC-02550

---
 message-index/messages/GHC-02550/index.md | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/message-index/messages/GHC-02550/index.md b/message-index/messages/GHC-02550/index.md
index aebf1682..c8b0aaa9 100644
--- a/message-index/messages/GHC-02550/index.md
+++ b/message-index/messages/GHC-02550/index.md
@@ -2,10 +2,11 @@
 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
+introduced: 9.6.1
 ---
 
-Quantified constraints are constraints which have their own constraints. The
+[Quantified constraints](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/exts/quantified_constraints.html)
+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