An unusual implicit any type discovered in a union with a generic type

When attempting to dynamically infer method parameters, I encounter an issue where an implicit 'any' type is inferred, even though a valid method definition is visible when hovering over the execute method. Renaming either method resolves the problem, but I prefer to keep them as they are. How can I achieve this?

type CommandBase<T extends BaseCommandInteraction> = ApplicationCommandData & {
  execute(interaction: T): unknown | Promise<unknown>
}

export type ContextMenuCommand = CommandBase<ContextMenuInteraction<"cached">> &
  (UserApplicationCommandData | MessageApplicationCommandData)

export type SlashCommand = CommandBase<CommandInteraction<"cached">> &
  ChatInputApplicationCommandData

type ExtraLegacyCommandData<TArgs> = {
  resolveArgs(args: Args, message: Message): TArgs
}

type LegacyCommand<TArgs> = {
  type: "LEGACY"
  name: string
  description: string
  execute(message: Message, args: TArgs): unknown | Promise<unknown>
  // eslint-disable-next-line @typescript-eslint/ban-types
} & (TArgs extends void ? {} : ExtraLegacyCommandData<TArgs>)

export type Command<TArgs> =
  | ContextMenuCommand
  | SlashCommand
  | LegacyCommand<TArgs>

export function createCommand<TArgs = void>(command: Command<TArgs>) {
  return command
}

export default createCommand({
  type: "LEGACY",
  name: "",
  description: "",
  execute(message) {
    // Type signature shows `execute(message: Message<boolean>, args: void): unknown`
    // But `Parameter 'message' implicitly has an 'any' type.`
  },
})

Link to playground

Edit: Inference works when defining both expected function parameters. Avoiding this step with added complexity:

type LegacyCommand<TArgs> = {
  type: "LEGACY"
  name: string
  description: string
  execute(
    message: Message,
    ...args: TArgs extends void ? [] : [TArgs]
  ): unknown | Promise<unknown>
  // eslint-disable-next-line @typescript-eslint/ban-types
} & (TArgs extends void ? {} : ExtraLegacyCommandData<TArgs>)

However, an alternative approach also fails:

// (method) execute(message: Message<boolean>): unknown
async execute(message) { // Parameter 'message' implicitly has an 'any' type.

Answer №1

I discovered that the issue I was facing was linked to this particular GitHub page. To resolve it, I added a specific constraint to my function and made slight modifications to my execute method.

type LegacyCommand<TArgs> = {
  type: "LEGACY"
  name: string
  description: string
  execute: (
    message: Message,
    // Only revealing arguments when an array of known type is provided as generic
    ...args: TArgs extends unknown[] ? [TArgs] : []
  ) => unknown | Promise<unknown>;
  // Checking for tuple generic input
} & (TArgs extends [unknown] ? ExtraLegacyCommandData<TArgs> : {});

// Adding explicit constraint
export function createCommand<TArgs extends unknown[] = []>(
  command: Command<TArgs>
) {
  return command;
}

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