Typescript partial type inference

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Type inference works based on an all or nothing principle. In the first case:

fn({ a: 1 })

has no generic type parameters provided so it will infer both:

  • B will be inferred as { a: number } based on the function argument;
  • and A will be inferred as unknown since it’s not used anywhere in the function.

In the second case:

fn<Fixed>({ a: 1 })

you have specified one of the generic types and that unfortunately means that type inference will not be used for the rest of the type parameters – therefore:

  • A is specified as Fixed;
  • B is not given so instead of inferring it it will default to {}.

As annoying as this is, it is just how TypeScript works. Your second example with two function calls is the usual workaround to this issue.

Related issue on GitHub.

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