Fix buglet in expandSynTyCon_maybewip/T19279

The fix for #17958, implemented in MR !2952, introduced a small bug
in GHC.Core.TyCon.expandSynTyCon_maybe, in the case of under-saturated
type synonyms.

This MR fixes the bug, very easy.

Fixes #19279

1 files changed, 6 insertions, 5 deletions

diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs
index 4db3167bd7..efa6cfbcf7 100644
--- a/compiler/GHC/Core/TyCon.hs
+++ b/compiler/GHC/Core/TyCon.hs
@@ -2257,13 +2257,14 @@ expandSynTyCon_maybe
                                   -- type of the synonym (not yet substituted)
                                   -- and any arguments remaining from the
                                   -- application
-
--- ^ Expand a type synonym application, if any
+-- ^ Expand a type synonym application
+-- Return Nothing if the TyCon is not a synonym,
+-- or if not enough arguments are supplied
 expandSynTyCon_maybe tc tys
   | SynonymTyCon { tyConTyVars = tvs, synTcRhs = rhs, tyConArity = arity } <- tc
-  = case tys of
-      [] -> Just ([], rhs, []) -- Avoid a bit of work in the case of nullary synonyms
-      _  -> case tys `listLengthCmp` arity of
+  = if arity == 0
+    then Just ([], rhs, tys)  -- Avoid a bit of work in the case of nullary synonyms
+    else case tys `listLengthCmp` arity of
               GT -> Just (tvs `zip` tys, rhs, drop arity tys)
               EQ -> Just (tvs `zip` tys, rhs, [])
               LT -> Nothing