Assignment of incompatible types missed by the compiler in combination with generic interface with function template parameter

[Davidiusdadi]

Bug Report

🔎 Search Terms

• template inference
• no type error for bad assignment
• using extends clause causes bad typecheck to pass / not to fail
• using extends T suppresses failing type check

🕗 Version & Regression Information

• compiler fails to detect incompatible function signature
• tested using versions 5.1, 5.1 nightly but also 1.8, 2.9 and 3.9

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo<T> {
  bar<P extends T>(payload: P): number;
  
  // without the extends we actually get a compiler error 
  // bar(payload: T): number;
}

type PayloadA = { valueA: number };
const foo_a: Foo<PayloadA> = {
    bar: (val) => val.valueA
};

type PayloadB = { valueB: string };
let foo_b: Foo<PayloadB> = {
    bar: (val) =>  val.valueB.length
};

const payload_a = { valueA: 42 } satisfies PayloadA
const payload_b = { valueB: 'test'} satisfies PayloadB


foo_a.bar(payload_a); // OK - valid
foo_a.bar(payload_b); // OK - does not compile
foo_b.bar(payload_b); // OK - valid
foo_b.bar(payload_a); // OK - does not compile


foo_b = foo_a; // ERROR - does compile BUT should NOT

foo_b.bar(payload_b) // this will throw at runtime because the above assignment

🙁 Actual behavior

The assignment foo_b = foo_a compiles without errors even though it should not, because Foo and Foo are not compatible (because their functions Foo['bar'] and Foo['bar'] each expect different parameters)

🙂 Expected behavior

The assignment foo_b = foo_a should give a compiler error as their signatures do not match.
Error message should be e.g:

TS2322: Type 'Foo<PayloadA>' is not assignable to type 'Foo<PayloadB>'.   Property 'valueB' is missing in type 'PayloadA' but required in type 'PayloadB'

Note

Interestingly we would get the expected behaviour if we used the bar(payload: T): number; instead of bar<P extends T>(payload: P): number;

[RyanCavanaugh]

This is the same root cause as the fix at #48092

You can add a variance annotation to Foo to work around this:

interface Foo<in T> {
  bar<P extends T>(payload: P): number;
}

[Davidiusdadi]

Thank you @RyanCavanaugh 🙏 for the quick response and the "variance annotation" tip.

Happy to have the confirmation the described issue is indeed a bug and not somehow feature.

I spent like 20min to look for a duplicate - would never have found the duplicates with the keywords I was using. 😅

Since the duplicates have so radically different wording I hope that this issue still helps others who encounter this bug. Therefore I'd not close this - plz go ahead to close though in case that is standard procedure.

Just for the mention: this also a duplicate of / related to #48070
Looking forward to #48092 getting fixed.

[typescript-bot]

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