Why is defining a constructor on @ObjectType class forbidden?

[maclockard]

The docs call out the fact that defining a constructor on an object type is forbidden:

Be aware that defining constructors is strictly forbidden and we shouldn't use them there, as TypeGraphQL creates instances of object type classes under the hood by itself.

source

However, I'm not sure I really understand why? I've used constructors for some @ObjectType classes and it didn't seem to have an affect.

Additionally, it seems like using a constructor for union types is encouraged: #1179 (comment)

[MichalLytek]

Because object instances are created multiple times in the pipeline and are called without any arguments.

[antontsvil]

Can you expand on under what circumstances a type is instantiated without arguments? I tried replicating this scenario and my constructors only ever seem to run once when I call them. Additionally, would switching to a pattern where constructors always null check arguments be sufficient? For example:

@InterfaceType()
class HasFoo {
  constructor(args?: Required<HasFoo>) {
    if (args) {
      this.foo = args.foo;
    }
  }

  @Field() foo!: string;
}

@ObjectType({ implements: HasFoo })
class Bar extends HasFoo {
  constructor(args?: Required<Bar & HasFoo>) {
    console.log('Instantiating bar', { args });
    super(args);
    if (args) {
      Object.assign(this, args);
    }
  }

  @Field(type => Int)
  public length!: number;

  @Field(type => ID)
  public name!: string;

  @Field(type => Int)
  public width!: number;
}

[MichalLytek]

Types are instantiated always when they are used as a resolver method parameter (like @Root or @Arg).