constraint of type parameter should make effect in method declaration

[bvanjoi]

🔎 Search Terms

"constraint", "method declaration"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "A method and a function property of the same type behave differently"

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250127#code/JYOwLgpgTgZghgYwgAgIIB4AqA+ZBvAKGWRgHtT0BVZCAD0hABMBnZHACloC5lKBKHnBABPAgF8CBGAFcQCMMFIgSABnZweGZmCigA5tgA0yAEab0IaQFsT0bH3xFip5AF5kcZAHovyZgAtSaQAbRmQoCAAHUigwGigoGNNpOMQwaThg4OFkEFIwADpJZ1MCslJ2AEYHH2RZZiCmcUkgA

💻 Code

interface A<T> {
  foo<U extends T>(x: U): any
}

function f0(a: A<string>, b: A<number>) {
    b = a // should report error but actually not.

    b.foo(1) // unsound
}

🙁 Actual behavior

check success

🙂 Expected behavior

b = a report number cannot assign to string.

Additional information about the issue

No response

[RyanCavanaugh]

Duplicate #55671 (see #55864)

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.