Consider HsExpanded expressions during SYB traversal

[ozkutuk]

Fixes #3574. Implements the proposed solution and adds the given reproduction steps as a test case.

[joyfulmantis]

Thanks for getting the patch out so quickly!

[joyfulmantis]

The problem with returning a Maybe instead of a List here is that you can only return one match from within the HsExpanded expression, even though there could be multiple matches. This example showcases this.

{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedRecordDot #-}
module Construction where

data MyRec = MyRec
  { foo :: Int
  , bar :: Int
  , baz :: Char
  }

data YourRec = YourRec
  { foo2 :: MyRec
  , bar2 :: Int
  , baz2 :: Char
  }

myRecExample = MyRec {..}
  where
    foo = 5
    bar = 6
    baz = 'a'

yourRecExample = YourRec {..}
    where
        foo2 = myRecExample
        bar2 = 5
        baz2 = 'a'

convertMe :: () -> Int
convertMe _ =
  (let MyRec{..} = myRecExample
       YourRec{..} = yourRecExample
    in foo2).foo

In this case, your code, would only offer to rewrite the first matched wildcard instead of both.
This example is very constructed, however, HsExpansion could possibly be used for more cases in the future, which would cause these sorts of edge cases to also appear more often.

[joyfulmantis]

ConPat can also occur inside an HsExpanded expression. For example, the constructed example above uses ConPat (because the wildcards occur in the let binding)

[joyfulmantis]

Actually thinking about it though, you shouldn't need to change this, because getRecCon should already, by calling collectRecords, return both RecordCon and ConPat matches. May want to point that out in the comments though.

[joyfulmantis]

Closed. Completed in #3750 .