code: public static function isTypeSubTypeOf(Schema $schema, $maybeSubType, $superType)
      {
          // Equivalent type is a valid subtype
          if ($maybeSubType === $superType) {
              return true;
          }
  
          // If superType is non-null, maybeSubType must also be nullable.
          if ($superType instanceof NonNull) {
              if ($maybeSubType instanceof NonNull) {
                  return self::isTypeSubTypeOf($schema, $maybeSubType->getWrappedType(), $superType->getWrappedType());
              }
  
              return false;
          }
  
          if ($maybeSubType instanceof NonNull) {
              // If superType is nullable, maybeSubType may be non-null.
              return self::isTypeSubTypeOf($schema, $maybeSubType->getWrappedType(), $superType);
          }
  
          // If superType type is a list, maybeSubType type must also be a list.
          if ($superType instanceof ListOfType) {
              if ($maybeSubType instanceof ListOfType) {
                  return self::isTypeSubTypeOf($schema, $maybeSubType->getWrappedType(), $superType->getWrappedType());
              }
  
              return false;
          }
  
          if ($maybeSubType instanceof ListOfType) {
              // If superType is not a list, maybeSubType must also be not a list.
              return false;
          }
  
          // If superType type is an abstract type, maybeSubType type may be a currently
          // possible object type.
          return Type::isAbstractType($superType) &&
              $maybeSubType instanceof ObjectType &&
              $schema->isPossibleType(
                  $superType,
                  $maybeSubType
              );
      }
docs: Provided a type and a super type, return true if the first type is either
  equal or a subset of the second super type (covariant).
  
  @param AbstractType $maybeSubType
  @param AbstractType $superType
  
  @return bool