SI-7987 Test case for "macro not expanded" error with implicits

Fixed, serendipitously, a handful of commits ago in 4a6882e772,
"SI-7944 FOUND: stray undetermined type params in vicinity of
implicits".
Merge pull request #3172 from retronym/ticket/7987

SI-7987 Test case for "macro not expanded" error with implicits