Refine ASSERT in buildPatSyn for the nullary case.

For a nullary pattern synonym we add an extra void argument to the
matcher in order to preserve laziness. The check in buildPatSyn
wasn't aware of this special case which was causing the assertion to
fail.

Reviewers: austin, simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2624

GHC Trac Issues: #12746

compiler/iface/BuildTyCl.hs

@@ -18,6 +18,7 @@ module BuildTyCl (
 import IfaceEnv
 import FamInstEnv( FamInstEnvs, mkNewTypeCoAxiom )
 import TysWiredIn( isCTupleTyConName )
+import TysPrim ( voidPrimTy )
 import DataCon
 import PatSyn
 import Var
@@ -247,7 +248,7 @@ buildPatSyn src_name declared_infix matcher@(matcher_id,_) builder
                  , pat_ty `eqType` substTy subst pat_ty1
                  , prov_theta `eqTypes` substTys subst prov_theta1
                  , req_theta `eqTypes` substTys subst req_theta1
-                 , arg_tys `eqTypes` substTys subst arg_tys1
+                 , compareArgTys arg_tys (substTys subst arg_tys1)
                  ])
             , (vcat [ ppr univ_tvs <+> twiddle <+> ppr univ_tvs1
                     , ppr ex_tvs <+> twiddle <+> ppr ex_tvs1
@@ -263,11 +264,19 @@ buildPatSyn src_name declared_infix matcher@(matcher_id,_) builder
     ((_:_:univ_tvs1), req_theta1, tau) = tcSplitSigmaTy $ idType matcher_id
     ([pat_ty1, cont_sigma, _], _)      = tcSplitFunTys tau
     (ex_tvs1, prov_theta1, cont_tau)   = tcSplitSigmaTy cont_sigma
-    (arg_tys1, _) = tcSplitFunTys cont_tau
+    (arg_tys1, _) = (tcSplitFunTys cont_tau)
     twiddle = char '~'
     subst = zipTvSubst (univ_tvs1 ++ ex_tvs1)
                        (mkTyVarTys (binderVars (univ_tvs ++ ex_tvs)))
 
+    -- For a nullary pattern synonym we add a single void argument to the
+    -- matcher to preserve laziness in the case of unlifted types.
+    -- See #12746
+    compareArgTys :: [Type] -> [Type] -> Bool
+    compareArgTys [] [x] = x `eqType` voidPrimTy
+    compareArgTys arg_tys matcher_arg_tys = arg_tys `eqTypes` matcher_arg_tys
+
+
 ------------------------------------------------------
 type TcMethInfo     -- A temporary intermediate, to communicate
                     -- between tcClassSigs and buildClass.

testsuite/tests/patsyn/should_compile/T12746.hs

@@ -0,0 +1,7 @@
+module T12746 where
+
+import T12746A
+
+foo a = case a of
+        Foo -> True
+        _ -> False

testsuite/tests/patsyn/should_compile/T12746A.hs

@@ -0,0 +1,5 @@
+{-# LANGUAGE ScopedTypeVariables, PatternSynonyms #-}
+module T12746A where
+
+pattern Foo :: Int
+pattern Foo = 0x00000001

testsuite/tests/patsyn/should_compile/all.T

@@ -61,3 +61,4 @@ test('T12484', normal, compile, [''])
 test('T11987', normal, multimod_compile, ['T11987', '-v0'])
 test('T12615', normal, compile, [''])
 test('T12698', normal, compile, [''])
+test('T12746', normal, multi_compile, ['T12746', [('T12746A.hs', '-c')],'-v0'])