[CSStep] Isolated conjunctions can't see outer solutions

All of the previously deduced solutions should be hidden
until isolated conjunction is successfully solved.

Author: xedin

Diff for: lib/Sema/CSStep.cpp

@@ -851,6 +851,8 @@ StepResult ConjunctionStep::resume(bool prevFailed) {
     if (prevFailed)
       return failConjunction();
 
+    // There could be a local ambiguity related to
+    // the current element, let's try to resolve it.
     if (Solutions.size() > 1) {
       filterSolutions(Solutions, /*minimize=*/true);
 
@@ -918,7 +920,7 @@ StepResult ConjunctionStep::resume(bool prevFailed) {
 
       // Now that all of the information from the conjunction has
       // been applied, let's attempt to solve the outer scope.
-      return suspend(std::make_unique<SplitterStep>(CS, Solutions));
+      return suspend(std::make_unique<SplitterStep>(CS, OuterSolutions));
     }
   }
 

Diff for: lib/Sema/CSStep.h

@@ -858,12 +858,25 @@ class ConjunctionStep : public BindingStep<ConjunctionElementProducer> {
   /// system step before conjunction step.
   Optional<SolverSnapshot> Snapshot;
 
+  /// A set of previously deduced solutions. This is used upon
+  /// successful solution of an isolated conjunction to introduce
+  /// all of the inferred information back into the outer context.
+  SmallVectorImpl<Solution> &OuterSolutions;
+
+  /// Solutions produced while attempting elements of an isolated conjunction.
+  ///
+  /// Note that this is what `BindingStep` is initialized with
+  /// in isolated mode.
+  SmallVector<Solution, 4> IsolatedSolutions;
+
 public:
   ConjunctionStep(ConstraintSystem &cs, Constraint *conjunction,
                   SmallVectorImpl<Solution> &solutions)
-      : BindingStep(cs, {cs, conjunction}, solutions),
+      : BindingStep(cs, {cs, conjunction},
+                    conjunction->isIsolated() ? IsolatedSolutions : solutions),
         BestScore(getBestScore()), CurrentScore(getCurrentScore()),
-        Conjunction(conjunction), AfterConjunction(erase(conjunction)) {
+        Conjunction(conjunction), AfterConjunction(erase(conjunction)),
+        OuterSolutions(solutions) {
     assert(conjunction->getKind() == ConstraintKind::Conjunction);
 
     // Make a snapshot of the constraint system state before conjunction.