Error span incorrect when `this` constraint on member function fails

🔎 Search Terms

• error span
• this: never
• this chain
• builder pattern

🕗 Version & Regression Information

• This is the behavior in every version I tried (including the latest 5.7.2)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241206#code/PQKhCgAIUhZBTAtgI3gJ0gMwK4DsDGALgJYD2ukyAhgM7wAmk+t8UIw4+ANrTZAGJ4iZXACEW9UdmJd66ADwBlQvAAOkeAA8Vuen1zYU6ALwBGAHyQA3lEg0Vq0wApCAC2I0AXJGVqN2+F0+U0gAfkg3D0hvXHgAN3QASm9BAhJycTpJaVkFACZLG0hiyDR4Qmw0Cki+WkgqXABPAG5bAF9bezU8l3cvHwd-HT1IPLCIvujIWIS0ZIEhdLEJKRk5NHkAZkLbYrKKqomouoaW9ttgYDsAa2J1LvVN-1UuYnxiQi5GiNJIOURyPY0FQVBN4Bo0GhSGhOg4ACy9DzeXzqLTDPhw8Y1KYzJIpRYiTIMVa5DYAVh2JVK5Uq1UmJyarWKHQ64FAEGgcCQqAwcSoaGIVGQXHB1CyTBYbA43F4kAAavzBcL4ETsmsFCihoERgYjGgzJS7A5TN4nJq0drguMnIlIMZLAqBUKRaqSet5AUcfEknbIE4bXbDXsaYdsQyzszbeGmUbuqbzQEgqNrbb7fLFc6VSscu7tl7Zqm-QG00USvtaUdarVGe0o9WI5BLjc7rHHs9Xu9Pt9CL9-oDCMDQW5wegoTDig84fHBhak5jwsWHRnla6cwoKfmfcYi6mg9SDnTjvWY206-Ua6z8P2sEI3ehfbEAO4LNKE7PqtA21pNqhcLiQUhMEgAADTBSFIAA6B5nESKCHB6RJgIiWk+GIXAe3PSA8FceAuFUHB-1HaFIEQeAaBoKgAHNWBwAg7zQOC1BgxjVAQliEUSVpOGvPk0Hoh94GfR0lRdd9SS-NkrmHCEx0gVQoVUdAvnqVRFP5PhyGU8hW0gOFwF4+iWOYh42MnL8gA

💻 Code

/**
 * Member function based case
 */
class FunctionBasedBuilder<Step extends number=1> {
  step1(this: Step extends 1 ? this : never): FunctionBasedBuilder<2> {
    return this as any;
  }
  step2(this: Step extends 2 ? this : never): FunctionBasedBuilder<3> {
    return this as any;
  }
  // skip step 3 explicitly to demonstrate the error
  step4(this: Step extends 4 ? this : never): FunctionBasedBuilder<5> {
    return this as any;
  }
}

/**
 * Member variable based case
 */
class VariableBasedBuilder<Step extends number=1> {
  step1: (Step extends 1 ? () => VariableBasedBuilder<2> : never) = (() => {
    return this as any;
  }) as any;
  step2: (Step extends 2 ? () => VariableBasedBuilder<3> : never) = (() => {
    return this as any;
  }) as any;
  // skip step 3 explicitly to demonstrate the error
  step4: (Step extends 4 ? () => VariableBasedBuilder<5> : never) = (() => {
    return this as any;
  }) as any;
}

const funcBuilder = new FunctionBasedBuilder();
// all of `foo.step1().step2()` turns into an unhelpful error message
funcBuilder.step1().step2().step4();

const varBuilder = new VariableBasedBuilder();
// the error properly appears only on step 4
varBuilder.step1().step2().step4();

🙁 Actual behavior

In the function case, the entire span from foo.step1().step2() gets marked as an error instead of only step4() which is what actually caused the error

🙂 Expected behavior

Just like in the member variable case, it would be better if the error was shown on step4()

Additional information about the issue

I think this ends up being equivalent to #28159 so I'm not sure it can be solved without introducing the "curse" mentioned in #60140

Apri il TypeScript Playground collegato e confronta i casi di builder basati su funzione e su variabile, concentrandoti sull’intervallo diagnostico per la chiamata finale a step4. Esamina la possibile relazione con le issue #28159 e #60140. Il lavoro è completato quando il caso basato su funzione evidenzia solo step4(), come il caso basato su variabile, senza introdurre regressioni nell’errore segnalato.