Fix optSemi type in Parser.ywip/optsemi-unsafe-coerce

The definition of 'optSemi' claimed it had type

([Located a],Bool)

Note that its production actually returns ([Located Token],Bool):

  : ';' { ([$1],True) }   -- $1 :: Located Token

Due to an infelicity in the implementation of 'happy -c', it effectively
resulted in 'unsafeCoerce :: Token -> a'.
See https://github.com/simonmar/happy/pull/134

If any consumer of 'optSemi' tried to instantiate 'a' to something not
representationally equal to 'Token', they would experience a segfault.

In addition to that, this definition made it impossible to compile Parser.y
without the -c flag (as it's reliant on this bug to cast 'Token' to 'forall
a. a').

1 files changed, 1 insertions, 2 deletions

diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
index ce5c523e6f..2d1e8f3c9b 100644
--- a/compiler/parser/Parser.y
+++ b/compiler/parser/Parser.y
@@ -1,4 +1,3 @@
-
 --                                                              -*-haskell-*-
 -- ---------------------------------------------------------------------------
 -- (c) The University of Glasgow 1997-2003
@@ -2580,7 +2579,7 @@ exp10 :: { LHsExpr GhcPs }
         | scc_annot exp        {% ams (sLL $1 $> $ HsSCC noExt (snd $ fst $ unLoc $1) (snd $ unLoc $1) $2)
                                       (fst $ fst $ unLoc $1) }
 
-optSemi :: { ([Located a],Bool) }
+optSemi :: { ([Located Token],Bool) }
         : ';'         { ([$1],True) }
         | {- empty -} { ([],False) }