Merge pull request #434 from haskellfoundation/ghc-00711

Document GHC-00711

Author: BinderDavid

message-index/messages/GHC-00711/doubleDeprecation/after/DoubleDeprecation.hs

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+module DoubleDeprecation where
+
+{-# DEPRECATED pi "pi is deprecated. Chosen approximation turned out to be too imprecise." #-}
+pi :: Int
+pi = 3

message-index/messages/GHC-00711/doubleDeprecation/before/DoubleDeprecation.hs

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+module DoubleDeprecation where
+
+{-# DEPRECATED pi "pi is deprecated." #-}
+{-# DEPRECATED pi "Chosen approximation turned out to be too imprecise." #-}
+pi :: Int
+pi = 3

message-index/messages/GHC-00711/doubleDeprecation/index.md

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+---
+title: Two deprecation warnings for same constant
+---
+
+In this example, two deprecation warnings have been given for the constant `pi`. This is not allowed, and can be fixed by just specifying one deprecation message.
+
+```
+messages/GHC-00711/doubleDeprecation/before/DoubleDeprecation.hs:4:16: error: [GHC-00711]
+    Multiple warning declarations for ‘pi’
+    also at  messages/GHC-00711/doubleDeprecation/before/DoubleDeprecation.hs:3:16-17
+  |
+4 | {-# DEPRECATED pi "Chosen approximation turned out to be too imprecise." #-}
+  |                ^^
+```

message-index/messages/GHC-00711/index.md

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+---
+title: Duplicate warning declarations
+summary: Duplicate deprecation warnings are illegal
+severity: error
+introduced: 9.6.1
+---
+
+A Haskell entity, such as a type or a top-level definition, can be declared to be deprecated using a pragma.
+It is, however, illegal to specify multiple deprecation warnings for the same entity.