private static CompoundInitialState createState(InferenceSession topLevelSession) {
    final PsiSubstitutor topInferenceSubstitutor =
        replaceVariables(topLevelSession.getInferenceVariables());
    final Map<PsiElement, InitialInferenceState> nestedStates =
        new LinkedHashMap<PsiElement, InitialInferenceState>();

    final InferenceSessionContainer copy =
        new InferenceSessionContainer() {
          @Override
          public PsiSubstitutor findNestedSubstitutor(
              PsiElement arg, @Nullable PsiSubstitutor defaultSession) {
            // for the case foo(bar(a -> m())): top level inference won't touch lambda "a -> m()"
            // for the case foo(a -> bar(b -> m())): top level inference would go till nested lambda
            // "b -> m()" and the state from top level could be found here by "bar(b -> m())"
            // but proceeding with additional constraints from saved point would produce new
            // expression constraints with different inference variables (could be found in
            // myNestedSessions)
            // which won't be found in the system if we won't reject stored sessions in such cases
            final PsiSubstitutor substitutor = super.findNestedSubstitutor(arg, null);
            if (substitutor != null) {
              return substitutor;
            }

            final InitialInferenceState state =
                nestedStates.get(PsiTreeUtil.getParentOfType(arg, PsiCall.class));
            if (state != null) {
              return state.getInferenceSubstitutor();
            }
            return super.findNestedSubstitutor(arg, defaultSession);
          }
        };
    final Map<PsiElement, InferenceSession> nestedSessions =
        topLevelSession.getInferenceSessionContainer().myNestedSessions;
    for (Map.Entry<PsiElement, InferenceSession> entry : nestedSessions.entrySet()) {
      nestedStates.put(
          entry.getKey(),
          entry
              .getValue()
              .createInitialState(
                  copy, topLevelSession.getInferenceVariables(), topInferenceSubstitutor));
    }

    PsiSubstitutor substitutor = PsiSubstitutor.EMPTY;
    for (InferenceVariable variable : topLevelSession.getInferenceVariables()) {
      final PsiType instantiation = variable.getInstantiation();
      if (instantiation != PsiType.NULL) {
        final PsiClass psiClass =
            PsiUtil.resolveClassInClassTypeOnly(topInferenceSubstitutor.substitute(variable));
        if (psiClass instanceof InferenceVariable) {
          substitutor = substitutor.put((PsiTypeParameter) psiClass, instantiation);
        }
      }
    }

    return new CompoundInitialState(substitutor, nestedStates);
  }
 void registerNestedSession(
     InferenceSession session, PsiType returnType, PsiExpression returnExpression) {
   final PsiSubstitutor callSession =
       findNestedSubstitutor(((PsiCallExpression) returnExpression).getArgumentList(), null);
   if (callSession == null) {
     final InferenceSession inferenceSession =
         ExpressionCompatibilityConstraint.reduceExpressionCompatibilityConstraint(
             session, returnExpression, returnType);
     if (inferenceSession != null && inferenceSession != session) {
       registerNestedSession(inferenceSession);
       session.propagateVariables(
           inferenceSession.getInferenceVariables(),
           inferenceSession.getRestoreNameSubstitution());
     }
   }
 }