Document GHC-90973 (#496)

Author: jhrcek

message-index/messages/GHC-90973/exportedModNotImported/after/ExportedModNotImported.hs

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+module ExportedModNotImported (module Data.List) where
+
+import Data.List

message-index/messages/GHC-90973/exportedModNotImported/before/ExportedModNotImported.hs

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+module ExportedModNotImported (module Data.List) where

message-index/messages/GHC-90973/exportedModNotImported/index.md

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+---
+title: Exporting module which has not been imported
+---
+
+```haskell
+ExportedModNotImported.hs:1:32: error:
+    The export item ‘module Data.List’ is not imported
+  |
+1 | module ExportedModNotImported (module Data.List) where
+  |
+```
+
+We're trying to reexport a module which has not been imported.
+In this case we fix it by adding the missing import statement.

message-index/messages/GHC-90973/index.md

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+---
+title: Exported module not imported
+summary: Export list contains a module that is not imported
+severity: error
+introduced: 9.6.1
+---
+
+An *Export List* of a Haskell module identifies the entities (functions, types, etc.) to be exported by a module declaration.
+The syntax for export lists is described in the [section 5.2](https://www.haskell.org/onlinereport/modules.html) of the **Haskell 98 Language and Libraries** report. Apart from exporting individual functions and data types, the specification also allows you to re-export entire modules as in this example:
+
+```haskell
+module My.List.Reexport (module Data.List) where
+
+import Data.List
+
+...
+```
+
+This tells GHC to include all the symbols exported by the module `Data.List` to also be exported by `My.List.Reexport` module.
+
+The error message `GHC-90973` occurs when module's export list contains a module export of a module, which hasn't been imported.
+
+The usual way to fix it is to either:
+1. add the missing module import
+2. remove the `module <MISSING-MODULE>` export from the export list