Confusing parser error in 8.6

Consider the following example:

f x =
  case x of
    A -> 'a'
   B -> 'b'

The problem here is that B is misaligned, which is quite a common mistake, especially in a bigger case block.

GHC reports the following error:

    Unexpected case expression in function application:
        case x of { A -> 'a' }
    You could write it with parentheses
    Or perhaps you meant to enable BlockArguments?

This is quite confusing, especially since the program won't parse, BlockArguments or not. My guess is that we are being a bit too eager with reporting the BlockArguments issue.

One way to work around this would be to just remember if we used BlockArguments while parsing, but only do the check that the extension is enabled after a successful parse.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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