[ConstraintSystem] Make sure that invalid state is always diagnosed via fallback diagnostic

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -5472,6 +5472,14 @@ void ConstraintSystem::diagnoseFailureFor(SolutionApplicationTarget target) {
   SWIFT_DEFER { setPhase(ConstraintSystemPhase::Finalization); };
 
   auto &DE = getASTContext().Diags;
+
+  // If constraint system is in invalid state always produce
+  // a fallback diagnostic that asks to file a bug.
+  if (inInvalidState()) {
+    DE.diagnose(target.getLoc(), diag::failed_to_produce_diagnostic);
+    return;
+  }
+
   if (auto expr = target.getAsExpr()) {
     if (auto *assignment = dyn_cast<AssignExpr>(expr)) {
       if (isa<DiscardAssignmentExpr>(assignment->getDest()))