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Cannot be used to index type in 3.9 (worked in 3.8) #38740
Description
Activity
The immediate cause of this is #29571. Previously, a type parameter declared with
extends anywas treated as justany, which effectively turned off type checking. For example, you could changeStuff[A][B][C]toStuff[A][42]['elephant']in your example with no error!We now treat
extends anythe same as having no constraint, meaning that things actually get checked. And, in your exampleAmight not include'Blog'andBmight not include'posts', so there is no guarantee that'filtering' | 'ordering'is a valid key forStuff[A][B]. You need to include another check to make it work:type Get1< A extends any, B extends any, C extends 'filtering' | 'ordering' > = A extends keyof Stuff ? B extends keyof Stuff[A] ? C extends keyof Stuff[A][B] ? Stuff[A][B][C] // Ok : never : never : never;
Reacted by Gerrit BirkelandAhh thanks Anders Hejlsberg (@ahejlsberg) I thought I tried that 🤦.
I also mistakenly thought that TS would statically analyze that
Stuffonly hadfilteringandordering. I guess though the only way for that to happen would be havingStuffcome through as a constrained generic.There's probably an obvious reason TS must/does behave this way, but I don't see it.
Anyways, getting off-topic here.
Thanks again Anders Hejlsberg (@ahejlsberg)!
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on Oct 21, 2025
TypeScript Version: 3.9.3
Search Terms:
cannot be used to index type
Code
2536
// A *self-contained* demonstration of the problem follows...Expected behavior:
TS 3.9 supports the same type logic as in 3.8 (see playground repro)
Actual behavior:
Breaks in 3.9 (our context: graphql-nexus/nexus#859)
Playground Link:
link