Merge pull request #427 from HugoPeters1024/GHC-55666

document GHC-55666

Author: david-christiansen

message-index/messages/GHC-55666/example/after/Bang_on_unlifted_type.hs

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+{-# LANGUAGE MagicHash #-}
+
+module Bang_on_unlifted_type where
+
+import GHC.Base (Int#)
+
+data T = MkT Int#

message-index/messages/GHC-55666/example/before/Bang_on_unlifted_type.hs

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+{-# LANGUAGE MagicHash #-}
+
+module Bang_on_unlifted_type where
+
+import GHC.Base (Int#)
+
+data T = MkT !Int#

message-index/messages/GHC-55666/example/index.md

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+---
+title: Bang (!) on unlifted type
+---
+
+```
+src/MyLib.hs:16:5: warning: [GHC-55666] [-Wredundant-strictness-flags]
+    * Strictness flag has no effect on unlifted type `Int#'
+    * In the definition of data constructor `MkT'
+      In the data type declaration for `T'
+   |
+   |   = MkT !Int#
+   |     ^^^^^^^
+```

message-index/messages/GHC-55666/index.md

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+---
+title: Strictness annotation on unlifted type
+summary: Using a strictness annotation (bang) on an unlifted type is redudant as unlifted values are strict by definition
+severity: warning
+flag: -Wredundant-strictness-flags
+introduced: GHC 9.6.1
+---
+
+A strictness annotation (also called a bang: `!`) can be used to denote that a value should not be evaluated lazily.
+In some cases this can lead to faster code because fewer heap allocations are required.
+Here it is used to mark a that a field in a datatype should not be evaluated lazily.
+
+However, values of unlifted types like `Int#` are strict by definition because they represent an actual value,
+not a pointer to a potentially unevaluated value (thunk).
+
+Therefore, adding strictness annotations to unlifted types or fields of such types is redundant.
+They should be omitted to avoid confusion.