Different Behavior of Overriding Global Interface

[sxzz]

Steps to reproduce

declare global {
  interface ImportAttributes {
    // error on tsgo, no error on TS5
    cache?: 'no'
  }
}

export {}

Behavior with typescript@5.9

No errors: https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEBzCB7ARlC8DeAoe8AlgHYAuIMAZlGAgJIC2ADijKQIKmkyFoCu5AM458BeAHpx8UoMQp4-RABp4xFKXgUYrIsXgAVAMoBWUQTA0AFiAD8ALngByNY9EBfXB9wgAHizY4bkA

Behavior with tsgo

https://tsgo.sxzz.dev/#eNp1UDFqxDAQ/MqyzTXmrkrjJgTSXCCkSDrLhSyvHQVZa1brI2D898g2DoEjIJCY2RntzIwdllWFg/XxrAkLbMkFKwR94MYGmE0E8FFJOusIrsPIok+q4ptJKe08wOUCmnqGZuoLiKxAIixZCB/vD/uIs+6THks4RT6tyGJiPibS92oJ84J1UaEmx7Hz/fkrcczrbP4GHQ+jDyRvo3qOyWB5/GxQrfSkK2SQUvbL7+IgB26nQP+QKcdwm1Jlol+Y0uumuq6xebzjedJnL7tn69Nfx709uy55JxNKHG70koPt/sfE0QbWdYEOS8y5b/kKNjesuPwAtV2JNA==

With one error:

main.ts(4,5): error TS2411: Property 'cache' of type '"no" | undefined' is not assignable to 'string' index type 'string'.

[jakebailey]

This type is declared in lib.es5.d.ts as:

interface ImportAttributes {
    [key: string]: string;
}

If you instead do this same pattern in a single file:

interface Example {
    [key: string]: string;
}

interface Example {
    cache?: 'no'
}

You get the same error in the old compiler.

Property 'cache' of type '"no" | undefined' is not assignable to 'string' index type 'string'.(2411)

Playground Link

I'm pretty sure this is a bugfix, if anything? Likely related to the hardening done in #2134.

[sxzz]

I see, thanks for your answer!