Wrong inference for defaulted generic in nested call

🔎 Search Terms

nested generic call inline inference contextual typing function calls default type parameter

🕗 Version & Regression Information

• I see the same issue from version 4.1.5 to nightly in TypeScript Playground

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260401#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwCsctUAeAFXhAA8MRVgBneRjGEgcwBp4BVK2vSYs2nHgDUBdBs1btUHeAF54AclUA+ABStYGAFzxyPIYd49GIAA4B+Q+ICUhgAYASAN7kAvh-E-3vF7OAFCgkLAIKOjYeAJW4BgU2gBuUBDIIIbkTvDu8Bg4AEIgWjTxmCDAWTnJxMDwXsHBZQlaRCRaqgASIBAQOKo8qgDqODAQwKoODgB0BcWdPX04o+OTDvAA9JvwAEZQwE1geKzwAO5YGAAWOMgYAMrWsFAFMMqExKiLvf2DaqsTKbBbbwUHwAB6Nma1HKGC0F2utweTxgLzGs3mJQAREt+gDgFiNiCODgcIdobC2p9vss-iMxoChmppnMiiVuj8cPB8VMtjsSWTgkA

💻 Code

declare function join<T extends string, U extends string, V extends string = ''>(start: T, end: U, sep?: V): `${T}${V}${U}`
declare function expect<T>(value: T): { toBe(expected: T): void }

expect(join('Hello', 'World')).toBe('HelloWorld') // bad

const withoutSeparator = join('Hello', 'World')
//    ^? const withoutSeparator: "HelloWorld"
expect(withoutSeparator).toBe("HelloWorld") // good

expect(join('Hello', 'World', ' ')).toBe('Hello World') // good

🙁 Actual behavior

TypeScript does not infer the return types correctly of a nested function call when an optional generic param is omitted, even if a default is provided. When the function call is not nested, it is inferred correctly. It is also inferred correctly when all params are provided.

🙂 Expected behavior

I would expect that both of these approaches would compile:

expect(join('Hello', 'World')).toBe('HelloWorld') // bad

const withoutSeparator = join('Hello', 'World')
expect(withoutSeparator).toBe("HelloWorld") // good

Additional information about the issue

There are very similar issues, such as #54184, #56714, and #62680 but they didn't quite match up to what I'm seeing here, particularly since the reproductions are quite a bit more complex.

Comienza ejecutando la reproducción vinculada en TypeScript Playground y compara la llamada anidada con las llamadas independiente y completamente especificada. Lee los issues relacionados #54184, #56714 y #62680 para conocer el contexto de inferencia existente. Se considera terminado cuando ambos ejemplos sin separador especificado compilen con el tipo literal esperado, mientras que el caso con el separador proporcionado siga siendo correcto.