Duplicate diagnostics

[gabritto]

🔎 Search Terms

duplicate diagnostics, duplicate errors

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240415#code/JYOwLgpgTgZghgYwgAgMIHsC2AHANsBOSAEwBUBPbCAITgGcUBvAKGTeQG06AuZOsKKADmAXV4BBWgwDczAL7NQkWIhQYc+QiQpUAItGAA3CMWQt2nHnwHCxycfsHHishUujwk9qU1bs4vPyCIEKuiuAeqvaORiZmfmwARoE2IWEIuPR0yD7xFjDo6ACEvOp4BEQmOjT0EOnoIPzIAB7IALzIGVnIMc7IEM2QIMTZuebsBcWlWOVaVZQQvSZhuBBgyOiJAFa8jMgAzCnBQshy7WYHvABEYAAWUBAQV6eym1sczSLSQA

💻 Code

interface ComplicatedTypeBase {
    [s: string]: ABase;
}
interface ComplicatedTypeDerived {
    [s: string]: ADerived;
}
interface ABase {
    a: string;
}
interface ADerived {
    b: string;
}
class Base {
    foo!: ComplicatedTypeBase;
}
const x = class Derived extends Base {
    foo!: ComplicatedTypeDerived;
}
let obj: { 3: string } = { 3: "three" };
obj[x];

🙁 Actual behavior

Two instances of the same error on line foo!: ComplicatedTypeDerived;:

• Property 'foo' in type 'Derived' is not assignable to the same property in base type 'Base'. Type 'ComplicatedTypeDerived' is not assignable to type 'ComplicatedTypeBase'.(2416)
• Property 'foo' in type 'Derived' is not assignable to the same property in base type 'Base'. Type 'ComplicatedTypeDerived' is not assignable to type 'ComplicatedTypeBase'. 'string' index signatures are incompatible. Property 'a' is missing in type 'ADerived' but required in type 'ABase'.(2416)

🙂 Expected behavior

A single instance of the error

Additional information about the issue

No response

[gabritto]

Found this when coming up with test cases for #57842.
The way I came up with it is to look at where we call getTypeOfExpression, and then create a program where we call getTypeOfExpression on an expression that produces an error that is dependent on the state of the checker, such that we end up with two different diagnostics, one when we check the expression regularly, another when we re-check the expression through a call to getTypeOfExpression.

Note this is not unique to checking class expressions, nor to the specific error code in the example: I can likely construct other types of program that exhibit this same problem.

[ahejlsberg]

I think our duplicate elimination machinery needs to be smarter about removing error messages without elaborations when otherwise identical error messages with elaborations exist for the same location.