Sema: Workaround for invalid logic in Solution::simplifyTypeForCodeCompletion()

Author: slavapestov

lib/Sema/ConstraintSystem.cpp

@@ -1806,8 +1806,15 @@ Type Solution::simplifyTypeForCodeCompletion(Type Ty) const {
     // variable representing the argument to retrieve protocol requirements from
     // it. Look for a ArgumentConversion constraint that allows us to retrieve
     // the argument type var.
-    for (auto argConstraint :
-         CS.getConstraintGraph()[typeVar].getConstraints()) {
+    auto &cg = CS.getConstraintGraph();
+
+    // FIXME: The type variable is not going to be part of the constraint graph
+    // at this point unless it was created at the outermost decision level;
+    // otherwise it has already been rolled back! Work around this by creating
+    // an empty node if one doesn't exist.
+    cg.addTypeVariable(typeVar);
+
+    for (auto argConstraint : cg[typeVar].getConstraints()) {
       if (argConstraint->getKind() == ConstraintKind::ArgumentConversion &&
           argConstraint->getFirstType()->getRValueType()->isEqual(typeVar)) {
         if (auto argTV =