@Contract("null -> null")
  public static @Nullable MethodSignature getFunction(@Nullable final PsiClass psiClass) {
    if (isPlainInterface(psiClass)) {
      return CachedValuesManager.getProjectPsiDependentCache(psiClass, LambdaUtil::calcFunction);
    }
    return null;
  }
@Contract("null -> null")
  public static @Nullable MethodSignature getFunction(@Nullable final PsiClass psiClass)