Native tsc: interface-only string index signatures no longer simplify under generic indexed access (regressed in #65059) #65127

🔎 Search Terms

isStringIndexSignatureOnlyType, indexed access, string index signature, Dict[K], conditional type simplification, #65059

🕗 Version & Regression Information

• This changed in Allow accessing named declared members during a type's resolution #65059 (059d4c03492), which added a permanent ObjectFlagsClassOrInterface|ObjectFlagsReference exclusion to isStringIndexSignatureOnlyTypeWorker
• Still present on current main (042e0fb9574)
• Classic typescript@6.0.3 accepts the same code

⏯ Playground Link

N/A (native vs classic divergence)

💻 Code

interface Dict { [x: string]: number }

type AsNumber<T> = T extends number ? T : never;

function takesNumber<K extends string>(x: AsNumber<Dict[K]>): number {
    return x;
}

function callIt<K extends string>(x: Dict[K]) {
    return takesNumber(x); // should be OK
}

tsc --strict --noEmit a.ts

Control: replacing Dict with type Anon = { [x: string]: number } still works on native.

🙁 Actual behavior

Native tsc @ 042e0fb9574:

a.ts(8,24): error TS2345: Argument of type 'Dict[K]' is not assignable to parameter of type 'AsNumber<Dict[K]>'.
  Type 'number' is not assignable to type 'AsNumber<Dict[K]>'.

Dict[K] is not simplified to number, so the conditional AsNumber<…> stays deferred.

🙂 Expected behavior

Classic 6.0.3: no errors. An interface that is only a string index signature should still be treated as string-index-signature-only for indexed-access simplification, same as an anonymous object type.

Additional information about the issue

#65059 changed:

// before
t.flags&TypeFlagsObject != 0 && !c.isGenericMappedType(t) && …

// after
t.flags&TypeFlagsObject != 0 && t.objectFlags&(ObjectFlagsClassOrInterface|ObjectFlagsReference) == 0 && !c.isGenericMappedType(t) && …

That exclusion was needed to avoid circularity while members are still resolving, but applying it to fully resolved interfaces/classes that truly have only a string index signature breaks getIndexedAccessTypeOrUndefined's simplification path:

if c.isStringIndexSignatureOnlyType(objectType) && … {
    indexType = c.stringType
}

Suggested direction: gate the exclusion on unresolved / currently-resolving types (e.g. MembersResolved == 0 or resolvingObjectTypeStack), not on ClassOrInterface/Reference forever.