Can't parse an empty GADT with “where”

[neongreen]

This code is accepted by GHC but not by HSE:

{-# LANGUAGE GADTs #-}
data SomeGADT a where

Here's a test case:

> let gadts = defaultParseMode {extensions = [EnableExtension GADTs]}

> parseDeclWithMode gadts "data SomeGADT a where\n"
ParseFailed (SrcLoc "<unknown>.hs" 2 1) "Parse error: virtual }"

I'm using haskell-src-exts from master (commit 1a3a7b2).

[mpickering]

I spent a long time debugging this today so will write down my progress.

After lexing the where token, the next production that we want to match is the open production which pushes a layout context. However, we want this layout context to be pushed before we lex the newline character as otherwise we push a layout context on the next line which causes the lexer to get very confused.

Happily, I could simply refactor the grammar so that there was a separate rule for the the empty case which didn't have to deal with layout. Unhappily, this broke the special case where the deriving clause as at the same indent as the GADT decls. This causes problems as the lexer inserts a semicolon before deriving which is then consumed by the gadtdecl rule before it can error and trigger the close rule to finish the layout context.

[adamschoenemann]

This bug also applies to empty where clauses in general (as in where-bindings) but I can only manifest it with two empty where clauses.
A definition such as

foo :: Int -> Int
foo x = z + b where
  y = x where
  z = y where
  b = 1

parses fine by GHC but is a parse-error for haskell-src-exts

[Melvar]

This also applies to associated data declarations in an instance (which are different in the AST), as I just found out.