Class validator: Use class member as decorator argument

You can create a custom validator like below:

import {
  ValidationOptions,
  registerDecorator,
  ValidationArguments,
  buildMessage,
} from 'class-validator';
/**
* Install validator package from npm. class-validator uses validator under the 
* hood
*/
import {isISO31661Alpha2,isPostalCode} from 'validator';

export function IsPostalCodeOf(
  property: string,
  validationOptions?: ValidationOptions,
) {
  // eslint-disable-next-line @typescript-eslint/ban-types
  return function(object: Object, propertyName: string) {
    registerDecorator({
      name: 'isPostalCodeOf',
      target: object.constructor,
      propertyName: propertyName,
      constraints: [property],
      options: validationOptions,
      validator: {
        validate(value: any, args: ValidationArguments) {
          // Getting the country code field from the argument.
          // countryCode field from SignupDto
          const [countryCodeField] = args.constraints;
          // Getting the value of the countryCode Field
          const countryCode = (args.object as any)[countryCodeField];
          // Checking if the country code is valid even though it is checked 
          // at class level 
          if (!isISO31661Alpha2(countryCode)) {
          // Invalid county code
            return false;
          }
          // Checks if the value (zip) belongs in the extracted countryCode 
          // field
          return isPostalCode(value,countryCode);
        },
        // Specifiy your error message here.
        defaultMessage: buildMessage(
          eachPrefix =>
            `${eachPrefix} $property must be a valid postal 
             code in the specified country `,
          validationOptions,
        ),
      },
    });
  };
}

Usage:

export class SignupDto {
  @IsEmail()
  email: string

  @IsString()
  password: string

  @IsISO31661Alpha2()
  countryCode: string

  @IsPostalCodeOf('countryCode')
  zip: string
}

You can use the Validate decorator and create a custom validation method. For example, assuming you have properties zip and countryCode on your dto:

@ValidatorConstraint({ name: 'isPostalCodeByCountryCode', async: false })
class IsPostalCodeByCountryCode implements ValidatorConstraintInterface {
  validate(zip: string, args: ValidationArguments) {
    // @ts-ignore
    return isPostalCode(zip, args.object.countryCode);
  }

  defaultMessage(args: ValidationArguments) {
    // @ts-ignore
    return `Invalid zip "${args.object.zip}" for country "${args.object.countryCode}"`;
  }
}

Usage:

  @Validate(IsPostalCodeByCountryCode)
  public zip: string;

Decorators aren't able to inherently use class properties inside of them, due to how decorators work in typescript. You could create a custom validation decorator to read the other properties of the class and validate the zip property correctly, but that could be a bit of work to get going.

So to answer your question:

Is it possible to use a class member as decorator argument?

No, it is not.