Added documentation and examples for GHC-59692: Duplicate Instances

Author: Qbxs

message-index/messages/GHC-59692/divergingInstances/after/DivergingInstances.hs

@@ -0,0 +1,11 @@
+module DivergingInstances where
+
+newtype Sum = Sum { getSum :: Int }
+
+instance Semigroup Sum where
+    (<>) x y = Sum (getSum x + getSum y)
+
+newtype Product = Product { getProduct :: Int }
+
+instance Semigroup Product where
+    (<>) x y = Product (getProduct x * getProduct y)

message-index/messages/GHC-59692/divergingInstances/before/DivergingInstances.hs

@@ -0,0 +1,7 @@
+module DivergingInstances where
+
+instance Semigroup Int where
+    (<>) = (+)
+
+instance Semigroup Int where
+    (<>) = (*)

message-index/messages/GHC-59692/index.md

@@ -0,0 +1,11 @@
+---
+title: Duplicate Instances
+summary: Multiple instances defined for the same type class and type.
+severity: error
+introduced: 9.6.1
+---
+
+For type class coherence, at most one instance may be defined for each type for the same type class.
+
+Identical instances should simply be removed.
+If for the same type diverging instances are required, circumventing this restriction is possible by introducing a `newtype` wrapper.