Given this program:
main = do f $ do a <- return 3 c <- do return 5
GHC previously gave this error message:
Main.hs:2:7: error: Parse error in pattern: do a <- return 3 c Possibly caused by a missing 'do'? | 2 | f $ do | ^^...
What happened is GHC considered the whole f $ do a <- return 3 c as a
pattern. When parsed as an expression it becomes an infix application of
($), and GHC checks left and right hand sides before checking if ($)
is a valid infix constructor name, and shows the first error it got.
If instead we first check if the infix op is valid in pattern context,
the error message becomes much clearer:
Main.hs:2:3: error: Parse error in pattern: f $ do a <- return 3 c Possibly caused by a missing 'do'? | 2 | f $ do | ^^^^^^...
This may not entirely fix Trac #11188 but I think it's an improvement.