Allow imports to reference multiple fields with the same name (#21625)

If a module `M` exports two fields `f` (using DuplicateRecordFields), we can
still accept

    import M (f)
    import M hiding (f)

and treat `f` as referencing both of them.  This was accepted in GHC 9.0, but gave
rise to an ambiguity error in GHC 9.2.  See #21625.

This patch also documents this behaviour in the user's guide, and updates the
test for #16745 which is now treated differently.

1 files changed, 8 insertions, 1 deletions

diff --git a/docs/users_guide/exts/duplicate_record_fields.rst b/docs/users_guide/exts/duplicate_record_fields.rst
index aa842e7b4f..0a7504c399 100644
--- a/docs/users_guide/exts/duplicate_record_fields.rst
+++ b/docs/users_guide/exts/duplicate_record_fields.rst
@@ -57,4 +57,11 @@ However, this would not be permitted, because ``x`` is ambiguous: ::
 
     module M (x) where ...
 
-The same restrictions apply on imports.
+For ``import`` statements, it is possible to import multiple fields with the
+same name, as well as importing individual fields as part of their datatypes.
+For example, the following imports are allowed: ::
+
+    import M (S(x))        -- imports the type S and the 'x' field of S (but not the field of T)
+    import M (x)           -- imports both 'x' fields
+    import M hiding (S(x)) -- imports everything except the type S and its 'x' field
+    import M hiding (x)    -- imports everything except the two 'x' fields