@NotNull
  public static JetType createCorrespondingExtensionFunctionType(
      @NotNull JetType functionType, @NotNull JetType receiverType) {
    assert KotlinBuiltIns.getInstance().isFunctionType(functionType);

    List<TypeProjection> typeArguments = functionType.getArguments();
    assert !typeArguments.isEmpty();

    List<JetType> arguments = Lists.newArrayList();
    // excluding the last type argument of the function type, which is the return type
    int index = 0;
    int lastIndex = typeArguments.size() - 1;
    for (TypeProjection typeArgument : typeArguments) {
      if (index < lastIndex) {
        arguments.add(typeArgument.getType());
      }
      index++;
    }
    JetType returnType = typeArguments.get(lastIndex).getType();
    return KotlinBuiltIns.getInstance()
        .getFunctionType(functionType.getAnnotations(), receiverType, arguments, returnType);
  }
  /**
   * Determines what constraint (supertype, subtype or equal) should be generated for type parameter
   * {@code T} in a constraint like (in this example subtype one): <br>
   * {@code MyClass<in/out/- A> <: MyClass<in/out/- B>}, where {@code MyClass<in/out/- T>} is
   * declared. <br>
   * The parameters description is given according to the example above.
   *
   * @param typeParameterVariance declared variance of T
   * @param subjectTypeProjection {@code in/out/- A}
   * @param constrainingTypeProjection {@code in/out/- B}
   * @param upperConstraintKind kind of the constraint {@code MyClass<...A> <: MyClass<...B>}
   *     (subtype in this example).
   * @return kind of constraint to be generated: {@code A <: B} (subtype), {@code A >: B}
   *     (supertype) or {@code A = B} (equal).
   */
  @NotNull
  private static ConstraintKind getTypeParameterConstraintKind(
      @NotNull Variance typeParameterVariance,
      @NotNull TypeProjection subjectTypeProjection,
      @NotNull TypeProjection constrainingTypeProjection,
      @NotNull ConstraintKind upperConstraintKind) {
    // If variance of type parameter is non-trivial, it should be taken into consideration to infer
    // result constraint type.
    // Otherwise when type parameter declared as INVARIANT, there might be non-trivial use-site
    // variance of a supertype.
    //
    // Example: Let class MyClass<T> is declared.
    //
    // If super type has 'out' projection:
    // MyClass<A> <: MyClass<out B>,
    // then constraint A <: B can be generated.
    //
    // If super type has 'in' projection:
    // MyClass<A> <: MyClass<in B>,
    // then constraint A >: B can be generated.
    //
    // Otherwise constraint A = B should be generated.

    Variance varianceForTypeParameter;
    if (typeParameterVariance != INVARIANT) {
      varianceForTypeParameter = typeParameterVariance;
    } else if (upperConstraintKind == SUPER_TYPE) {
      varianceForTypeParameter = constrainingTypeProjection.getProjectionKind();
    } else if (upperConstraintKind == SUB_TYPE) {
      varianceForTypeParameter = subjectTypeProjection.getProjectionKind();
    } else {
      varianceForTypeParameter = INVARIANT;
    }

    return getTypeParameterConstraintKind(varianceForTypeParameter, upperConstraintKind);
  }
  private void addConstraint(
      @NotNull ConstraintKind constraintKind,
      @NotNull JetType subjectType,
      @Nullable JetType constrainingType,
      @NotNull ConstraintPosition constraintPosition) {

    if (constrainingType == TypeUtils.NO_EXPECTED_TYPE
        || constrainingType == DONT_CARE
        || constrainingType == CANT_INFER) {
      return;
    }

    if (constrainingType == null
        || (ErrorUtils.isErrorType(constrainingType)
            && constrainingType != PLACEHOLDER_FUNCTION_TYPE)) {
      hasErrorInConstrainingTypes = true;
      return;
    }

    assert subjectType != TypeUtils.NO_EXPECTED_TYPE
        : "Subject type shouldn't be NO_EXPECTED_TYPE (in position " + constraintPosition + " )";
    if (ErrorUtils.isErrorType(subjectType)) return;

    DeclarationDescriptor subjectTypeDescriptor =
        subjectType.getConstructor().getDeclarationDescriptor();

    KotlinBuiltIns kotlinBuiltIns = KotlinBuiltIns.getInstance();
    if (constrainingType == PLACEHOLDER_FUNCTION_TYPE) {
      if (!kotlinBuiltIns.isFunctionOrExtensionFunctionType(subjectType)) {
        if (subjectTypeDescriptor instanceof TypeParameterDescriptor
            && typeParameterConstraints.get(subjectTypeDescriptor) != null) {
          // a constraint binds type parameter and any function type, so there is no new info and no
          // error
          return;
        }
        errorConstraintPositions.add(constraintPosition);
      }
      return;
    }

    // todo temporary hack
    // function literal without declaring receiver type { x -> ... }
    // can be considered as extension function if one is expected
    // (special type constructor for function/ extension function should be introduced like
    // PLACEHOLDER_FUNCTION_TYPE)
    if (constraintKind == SUB_TYPE
        && kotlinBuiltIns.isFunctionType(constrainingType)
        && kotlinBuiltIns.isExtensionFunctionType(subjectType)) {
      constrainingType = createCorrespondingExtensionFunctionType(constrainingType, DONT_CARE);
    }

    DeclarationDescriptor constrainingTypeDescriptor =
        constrainingType.getConstructor().getDeclarationDescriptor();

    if (subjectTypeDescriptor instanceof TypeParameterDescriptor) {
      TypeParameterDescriptor typeParameter = (TypeParameterDescriptor) subjectTypeDescriptor;
      TypeConstraintsImpl typeConstraints = typeParameterConstraints.get(typeParameter);
      if (typeConstraints != null) {
        if (TypeUtils.dependsOnTypeParameterConstructors(
            constrainingType, Collections.singleton(DONT_CARE.getConstructor()))) {
          return;
        }
        if (subjectType.isNullable() && constrainingType.isNullable()) {
          constrainingType = TypeUtils.makeNotNullable(constrainingType);
        }
        typeConstraints.addBound(constraintKind, constrainingType);
        return;
      }
    }
    if (constrainingTypeDescriptor instanceof TypeParameterDescriptor) {
      assert typeParameterConstraints.get(constrainingTypeDescriptor) == null
          : "Constraining type contains type variable " + constrainingTypeDescriptor.getName();
    }
    if (constraintKind == SUB_TYPE && kotlinBuiltIns.isNothingOrNullableNothing(constrainingType)) {
      // following constraints are always true:
      // 'Nothing' is a subtype of any type
      if (!constrainingType.isNullable()) return;
      // 'Nothing?' is a subtype of nullable type
      if (subjectType.isNullable()) return;
    }
    if (!(constrainingTypeDescriptor instanceof ClassDescriptor)
        || !(subjectTypeDescriptor instanceof ClassDescriptor)) {
      errorConstraintPositions.add(constraintPosition);
      return;
    }
    switch (constraintKind) {
      case SUB_TYPE:
        {
          if (kotlinBuiltIns.isNothingOrNullableNothing(constrainingType)) break;
          JetType correspondingSupertype =
              TypeCheckingProcedure.findCorrespondingSupertype(constrainingType, subjectType);
          if (correspondingSupertype != null) {
            constrainingType = correspondingSupertype;
          }
          break;
        }
      case SUPER_TYPE:
        {
          if (kotlinBuiltIns.isNothingOrNullableNothing(subjectType)) break;
          JetType correspondingSupertype =
              TypeCheckingProcedure.findCorrespondingSupertype(subjectType, constrainingType);
          if (correspondingSupertype != null) {
            subjectType = correspondingSupertype;
          }
        }
      case EQUAL: // nothing
    }
    if (constrainingType.getConstructor() != subjectType.getConstructor()) {
      errorConstraintPositions.add(constraintPosition);
      return;
    }
    TypeConstructor typeConstructor = subjectType.getConstructor();
    List<TypeProjection> subjectArguments = subjectType.getArguments();
    List<TypeProjection> constrainingArguments = constrainingType.getArguments();
    List<TypeParameterDescriptor> parameters = typeConstructor.getParameters();
    for (int i = 0; i < subjectArguments.size(); i++) {
      Variance typeParameterVariance = parameters.get(i).getVariance();
      TypeProjection subjectArgument = subjectArguments.get(i);
      TypeProjection constrainingArgument = constrainingArguments.get(i);

      ConstraintKind typeParameterConstraintKind =
          getTypeParameterConstraintKind(
              typeParameterVariance, subjectArgument, constrainingArgument, constraintKind);

      addConstraint(
          typeParameterConstraintKind,
          subjectArgument.getType(),
          constrainingArgument.getType(),
          constraintPosition);
    }
  }