[CSStep] Reset scope counter before attempting conjunction step

Each conjunction step should be executed in isolation from outer
constraint system state, which should also include scope counter
because otherwise, e.g. for large closures, solver might stop
prematurely since all of the previous statements/expressions would
contribute to the scope total.

Author: xedin

Diff for: lib/Sema/CSStep.cpp

@@ -816,6 +816,10 @@ bool ConjunctionStep::attempt(const ConjunctionElement &element) {
   // by dropping all scoring information.
   CS.CurrentScore = Score();
 
+  // Reset the scope counter to avoid "too complex" failures
+  // when closure has a lot of elements in the body.
+  CS.CountScopes = 0;
+
   auto success = element.attempt(CS);
 
   // If element attempt has failed, mark whole conjunction

Diff for: lib/Sema/CSStep.h

@@ -857,6 +857,10 @@ class ConjunctionStep : public BindingStep<ConjunctionElementProducer> {
   /// The score established before conjunction is attempted.
   Score CurrentScore;
 
+  /// The number of constraint solver scopes already explored
+  /// before accepting this conjunction.
+  llvm::SaveAndRestore<unsigned> OuterScopeCount;
+
   /// Conjunction constraint associated with this step.
   Constraint *Conjunction;
   /// Position of the conjunction in the inactive constraints
@@ -889,8 +893,8 @@ class ConjunctionStep : public BindingStep<ConjunctionElementProducer> {
       : BindingStep(cs, {cs, conjunction},
                     conjunction->isIsolated() ? IsolatedSolutions : solutions),
         BestScore(getBestScore()), CurrentScore(getCurrentScore()),
-        Conjunction(conjunction), AfterConjunction(erase(conjunction)),
-        OuterSolutions(solutions) {
+        OuterScopeCount(cs.CountScopes, 0), Conjunction(conjunction),
+        AfterConjunction(erase(conjunction)), OuterSolutions(solutions) {
     assert(conjunction->getKind() == ConstraintKind::Conjunction);
 
     // Make a snapshot of the constraint system state before conjunction.