GHC matches -- as a varsym when lexing a qvarsym

GHC seems only to exclude the line comment marker -- when lexing a varsym and not a qvarsym.

This can be observed in the following code

module Demo where

x Demo.-- y = (x,y)

Generating this surprising error message

/Users/emertens/Desktop/Demo.hs:3:3: error:
    Qualified name in binding position: Demo.--

and excludes this definition perhaps?

example f = Just.-- comment
            f

Which might either have lexed as Just . -- comment, or it might have lexed as Just.- -, but it probably shouldn't be the case that it's a qvarsym.

At a minimum, the Haskell Report specifically excludes -- from the varsym rule, so it can't support the qvarsym rule match that it is now.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import parser labels

It looks like the lexer does absolutely no validation for anything that is qualified. Things like Prelude.|, Prelude.->, Prelude.data, etc. all make it all the way through to the renamer.

Most of these should (according to the report) probably lex as 3 or more separate tokens. Alternatively (and I prefer this), we could just report a lexer error. The functions for validating qualified symbols, constructors, etc. are already defined in basicTypes/Lexeme.hs, so there isn't a much work to be done here.

I'm also volunteering to implement either of these options. :)

Causes problems in Haddock too, see https://github.com/haskell/haddock/issues/952.

Trac metadata

Trac field	Value
CC	- → harpocrates

added Pnormal label

mentioned in commit haddock@2a2020c5