Fix incorrect ambiguity error on identically-named data constructors

Given multiple in-scope constructors with the same name, say `A`, and a
function of type `A -> Int`, say, the compiler reports both a "type `A`
is not in scope" and (incorrectly) an ambiguity error. The latter
shouldn't be there if `DataKinds` isn't enabled.

This issue was recommended to me by @mpickering as a suitable first
task, and the fix was also outlined in the original Trac ticket. It
involved a simple reordering of the steps taken in `lookup_demoted` in
`RnEnv.hs`. The fix is to make the `DataKinds` check happen earlier,
ensuring that the ambiguity check doesn't happen at all if we know the
constructors couldn't have been promoted.

Signed-off-by: Soham Chowdhury <chow.soham@gmail.com>

Reviewers: mpickering, austin, bgamari

Reviewed By: mpickering, bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13568

Differential Revision: https://phabricator.haskell.org/D3547

1 files changed, 11 insertions, 10 deletions

diff --git a/compiler/rename/RnEnv.hs b/compiler/rename/RnEnv.hs
index 570c6c0b91..12c8557b96 100644
--- a/compiler/rename/RnEnv.hs
+++ b/compiler/rename/RnEnv.hs
@@ -706,16 +706,17 @@ lookup_demoted rdr_name dflags
   | Just demoted_rdr <- demoteRdrName rdr_name
     -- Maybe it's the name of a *data* constructor
   = do { data_kinds <- xoptM LangExt.DataKinds
-       ; mb_demoted_name <- lookupOccRn_maybe demoted_rdr
-       ; case mb_demoted_name of
-           Nothing -> unboundNameX WL_Any rdr_name star_info
-           Just demoted_name
-             | data_kinds ->
-             do { whenWOptM Opt_WarnUntickedPromotedConstructors $
-                  addWarn (Reason Opt_WarnUntickedPromotedConstructors)
-                          (untickedPromConstrWarn demoted_name)
-                ; return demoted_name }
-             | otherwise  -> unboundNameX WL_Any rdr_name suggest_dk }
+       ; if data_kinds
+            then do { mb_demoted_name <- lookupOccRn_maybe demoted_rdr
+                    ; case mb_demoted_name of
+                        Nothing -> unboundNameX WL_Any rdr_name star_info
+                        Just demoted_name ->
+                          do { whenWOptM Opt_WarnUntickedPromotedConstructors $
+                               addWarn
+                                 (Reason Opt_WarnUntickedPromotedConstructors)
+                                 (untickedPromConstrWarn demoted_name)
+                             ; return demoted_name } }
+            else unboundNameX WL_Any rdr_name suggest_dk }
 
   | otherwise
   = reportUnboundName rdr_name