Typescript function overload implicitly has any type

type UpdateConsumerT = {
  obj: {
    min_quantity: number
    fixed_price: number
    date_start: string | boolean
    date_end: string | boolean
  }
}

type UpdateProducerT = {
  obj: {
    min_qty: number
    price: number
    date_start: string | boolean
    date_end: string | boolean
  }
}

export async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateConsumerT,
  isProduction: false
): Promise<number>
export async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateProducerT,
  isProduction: true
): Promise<number>
export async function updatePricelistItem(
  pricelist_item_id,
  obj,
  isProduction = false
) { ... }

I have this function, and i try to use overloads

But for some reason it says

> Parameter 'pricelist_item_id' implicitly has an 'any' type.ts(7006)

My question is, what am i doing wrong here?

You will need to explicitly specify the parameter types and return type on the implementation:

type UpdateConsumerT = {
  obj: {
    min_quantity: number
    fixed_price: number
    date_start: string | boolean
    date_end: string | boolean
  }
}

type UpdateProducerT = {
  obj: {
    min_qty: number
    price: number
    date_start: string | boolean
    date_end: string | boolean
  }
}

async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateConsumerT,
  isProduction: false
): Promise<number>
async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateProducerT,
  isProduction: true
): Promise<number>
async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateProducerT | UpdateConsumerT,
  isProduction = false
): Promise<number> { return new Promise(() => {}) }

Playground

Overload function implementations do not currently have their parameter types inferred contextually from the call signatures. There's a longstanding open feature request for this at microsoft/TypeScript#25352, but for now it's not part of the language.

Until and unless that gets implemented, you will need to manually annotate the function implementation's parameters:

export async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateConsumerT,
  isProduction: false
): Promise<number>
export async function updatePricelistItem(
  pricelist_item_id: number,
  obj: UpdateProducerT,
  isProduction: true
): Promise<number>

export async function updatePricelistItem(
  pricelist_item_id: number, // <-- annotate
  obj: UpdateConsumerT | UpdateProducerT, // <-- annotate
  isProduction = false // this is inferred as boolean from false
) {
  return null!; // suppress errors
}

Do note that because your call signatures return the same type for each call, you don't need to use overloads. Instead you can use a rest parameter whose type is a union of tuple types:

export async function updatePricelistItem(
  pricelist_item_id: number,
  ...[obj, isProduction]:
    [obj: UpdateConsumerT, isProduction: false] |
    [obj: UpdateProducerT, isProduction: true]
): Promise<number> {
  return null!; // suppress errors
}

declare const consumer: UpdateConsumerT;
declare const producer: UpdateProducerT;
updatePricelistItem(123, consumer, false); // okay
updatePricelistItem(123, producer, true); // okay
updatePricelistItem(123, consumer, true); // error

This approach is even better for your implementation because that rest parameter is a discriminated union with isProduction as the discriminant, so you can use control flow analysis on the destructured parameters to narrow obj based on isProduction:

export async function updatePricelistItem(
  pricelist_item_id: number,
  ...[obj, isProduction]:
    [obj: UpdateConsumerT, isProduction: false] |
    [obj: UpdateProducerT, isProduction: true]
): Promise<number> {
  if (isProduction) {
    obj.obj.price // okay, known to be UpdateProducerT
  } else {
    obj.obj.fixed_price // okay, known to be UpdateConsumerT
  }
  return null!;
}

Playground link to code