[ConstraintSystem] Add InvalidState flag to indicate that any attempt to solve should fail

As soon as `InvalidState` flag is set solving of the constraint
system as aborted and all subsequent calls to `solveImpl` would
produce no solutions.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -2141,6 +2141,11 @@ class ConstraintSystem {
   /// explored.
   size_t MaxMemory = 0;
 
+  /// Flag to indicate to the solver that the system is in invalid
+  /// state and it shouldn't proceed but instead produce a fallback
+  /// diagnostic.
+  bool InvalidState = false;
+
   /// Cached member lookups.
   llvm::DenseMap<std::pair<Type, DeclNameRef>, Optional<LookupResult>>
     MemberLookups;
@@ -2640,6 +2645,11 @@ class ConstraintSystem {
     Phase = newPhase;
   }
 
+  /// Check whether constraint system is in valid state e.g.
+  /// has left-over active/inactive constraints which should
+  /// have been simplified.
+  bool inInvalidState() const { return InvalidState; }
+
   /// Cache the types of the given expression and all subexpressions.
   void cacheExprTypes(Expr *expr) {
     bool excludeRoot = false;

lib/Sema/CSSolver.cpp

@@ -1384,6 +1384,13 @@ void ConstraintSystem::solveImpl(SmallVectorImpl<Solution> &solutions) {
   if (failedConstraint)
     return;
 
+  // Attempt to solve a constraint system already in an invalid
+  // state should be immediately aborted.
+  if (inInvalidState()) {
+    solutions.clear();
+    return;
+  }
+
   // Allocate new solver scope, so constraint system
   // could be restored to its original state afterwards.
   // Otherwise there is a risk that some of the constraints
@@ -1426,6 +1433,14 @@ void ConstraintSystem::solveImpl(SmallVectorImpl<Solution> &solutions) {
     // or error, which means that current path is inconsistent.
     {
       auto result = advance(step.get(), prevFailed);
+
+      // If execution of this step let constraint system in an
+      // invalid state, let's drop all of the solutions and abort.
+      if (inInvalidState()) {
+        solutions.clear();
+        return;
+      }
+
       switch (result.getKind()) {
       // It was impossible to solve this step, let's note that
       // for followup steps, to propogate the error.

lib/Sema/CSStep.cpp

@@ -384,6 +384,7 @@ StepResult ComponentStep::take(bool prevFailed) {
       getDebugLogger() << ")\n";
     }
 
+    CS.InvalidState = true;
     return finalize(/*isSuccess=*/false);
 #endif
   }
@@ -400,6 +401,7 @@ StepResult ComponentStep::take(bool prevFailed) {
         getDebugLogger() << ")\n";
       }
 
+      CS.InvalidState = true;
       return finalize(/*isSuccess=*/false);
 #endif
     }