Compiler allows narrower method signature than implemented interface about typescript HOT 7 CLOSED

Rare MartinJohns L 😅

Anyway I just updated the FAQ with a new entry about method bivariance; see https://github.com/microsoft/TypeScript/wiki/FAQ#why-method-bivariance

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That's not quite the right part of that document, see https://github.com/microsoft/TypeScript/wiki/FAQ#:~:text=A%20method%20and%20a%20function%20property%20of%20the%20same%20type%20behave%20differently.

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@RyanCavanaugh The new FAQ is missing a TOC, that threw me off and made finding things a lot harder.

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This is working as intended and mentioned in the FAQ: https://github.com/microsoft/TypeScript/wiki/FAQ#parameter-arity-variance-is-correct

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Thank you @MartinJohns for your quick reply.
I'm confused though, as my case is not about difference in arity but rather in the actual type of a positional method argument.
My apologies if I'm still overlooking a by-design behavior.

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ℹ️ @CComparon it works as you expect if you use the property syntax for class methods

class B {
  f = (a: number) => {}
}

class C { // same result when using an interface
  f = (a: number | string) => {}
}

async function main(): Promise<void> {
  const b: C = new B(); // Error: Type 'B' is not assignable to type 'C'... Type 'string' is not assignable to type 'number'.
  console.log(b.f(5));
  console.log(b.f("5"));
}

TS Playground

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Thanks both for pointing me to the rationale behind this language tradeoff.
Cheers

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