Add a HsExplicitFlag to SpliceDecl, to improve Trac #4042

The issue here is that 

    g :: A -> A
    f
    data A = A

is treated as if you'd written $(f); that is the call of
f is a top-level Template Haskell splice.  This patch 
makes sure that we *first* check the -XTemplateHaskellFlag
and bleat about a parse error if it's off.  Othewise we
get strange seeing "A is out of scope" errors.

1 files changed, 6 insertions, 2 deletions

diff --git a/compiler/hsSyn/HsDecls.lhs b/compiler/hsSyn/HsDecls.lhs
index 0038ebed0a..baf6eca76f 100644
--- a/compiler/hsSyn/HsDecls.lhs
+++ b/compiler/hsSyn/HsDecls.lhs
@@ -232,11 +232,15 @@ instance OutputableBndr name => Outputable (HsGroup name) where
 	  ppr_ds [] = empty
 	  ppr_ds ds = blankLine $$ vcat (map ppr ds)
 
-data SpliceDecl id = SpliceDecl (Located (HsExpr id))	-- Top level splice
+data SpliceDecl id 
+  = SpliceDecl			-- Top level splice
+        (Located (HsExpr id))
+        HsExplicitFlag		-- Explicit <=> $(f x y)
+				-- Implicit <=> f x y,  i.e. a naked top level expression
     deriving (Data, Typeable)
 
 instance OutputableBndr name => Outputable (SpliceDecl name) where
-   ppr (SpliceDecl e) = ptext (sLit "$") <> parens (pprExpr (unLoc e))
+   ppr (SpliceDecl e _) = ptext (sLit "$") <> parens (pprExpr (unLoc e))
 \end{code}