Keep the bindings local during defaultCallStacks

defaultCallStacks generates evidence bindings for call stacks,
but wasn't setting the binding site correctly.  As a result
they were simply discarded in the case of pattern synonyms,
giving rise to Trac #12489.

The fix is easy; and I added an ASSERT to catch the error earlier.

compiler/typecheck/TcPatSyn.hs

@@ -176,8 +176,10 @@ tcCheckPatSynDecl psb@PSB{ psb_id = lname@(L _ name), psb_args = details
 
        -- Solve the constraints now, because we are about to make a PatSyn,
        -- which should not contain unification variables and the like (Trac #10997)
+       ; empty_binds <- simplifyTop (mkImplicWC implics)
+
        -- Since all the inputs are implications the returned bindings will be empty
-       ; _ <- simplifyTop (mkImplicWC implics)
+       ; MASSERT2( isEmptyBag empty_binds, ppr empty_binds )
 
        -- ToDo: in the bidirectional case, check that the ex_tvs' are all distinct
        -- Otherwise we may get a type error when typechecking the builder,

compiler/typecheck/TcSMonad.hs

@@ -16,7 +16,7 @@ module TcSMonad (
     TcS, runTcS, runTcSDeriveds, runTcSWithEvBinds,
     failTcS, warnTcS, addErrTcS,
     runTcSEqualities,
-    nestTcS, nestImplicTcS,
+    nestTcS, nestImplicTcS, setEvBindsTcS,
 
     runTcPluginTcS, addUsedGREs, deferTcSForAllEq,
 
@@ -2487,6 +2487,10 @@ checkForCyclicBinds ev_binds
             -- Note [Deterministic SCC] in Digraph.
 #endif
 
+setEvBindsTcS :: Maybe EvBindsVar -> TcS a -> TcS a
+setEvBindsTcS m_ref (TcS thing_inside)
+ = TcS $ \ env -> thing_inside (env { tcs_ev_binds = m_ref })
+
 nestImplicTcS :: Maybe EvBindsVar -> TyCoVarSet -- bound in this implication
               -> TcLevel -> TcS a
               -> TcS (a, TyCoVarSet)  -- also returns any vars used when filling

compiler/typecheck/TcSimplify.hs

@@ -170,9 +170,12 @@ defaultCallStacks wanteds
   handle_simples simples
     = catBagMaybes <$> mapBagM defaultCallStack simples
 
-  handle_implic implic = do
-    wanteds <- defaultCallStacks (ic_wanted implic)
-    return (implic { ic_wanted = wanteds })
+  handle_implic implic
+    = do { wanteds <- setEvBindsTcS (ic_binds implic) $
+                      -- defaultCallStack sets a binding, so
+                      -- we must set the correct binding group
+                      defaultCallStacks (ic_wanted implic)
+         ; return (implic { ic_wanted = wanteds }) }
 
   defaultCallStack ct
     | Just _ <- isCallStackPred (ctPred ct)

testsuite/tests/patsyn/should_compile/T12489.hs

+{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
+
+module T12489 where
+pattern P :: a -> b
+pattern P a <- (undefined -> a)

testsuite/tests/patsyn/should_compile/all.T

@@ -58,3 +58,4 @@ test('T12094', normal, compile, [''])
 test('T11977', normal, compile, [''])
 test('T12108', normal, compile, [''])
 test('T12484', normal, compile, [''])
+test('T12489', normal, compile, [''])