[0.321 Rust port] Internal compiler panic (EInvalidObjectKit at flow_js_utils.rs:1297) on recursive homomorphic mapped type (react-hook-form DeepRequired)

[matclayton]

Again sorry for the Claude written issue, but this came out of the automate update, will try and validate this and find more details later on when I have time. Sorry figured it was better to report this now, than hold it back until I can fully understand it. Feel free to close if needed.

Flow version: 0.321.0 (regression from 0.318.0, which checked the same code with no errors)

Summary

The Rust-backed checker panics (internal compiler error, brings the server down with exit 78) instead of emitting a type error when checking react-hook-form's DeepRequired<T> type. The crash fires whenever a file instantiates an RHF form whose value type contains primitive fields.

The proximate cause is the recursive homomorphic mapped type bottoming out on a primitive: DeepRequired<T[K]> is evaluated where T[K] is string/boolean/etc., and the object-kit path panics rather than producing a graceful error. A simplified standalone version of the same shape yields a clean not-an-object error instead of crashing, so the panic appears specific to the full DeepRequired + FieldErrorsImpl<DeepRequired<T>> + template-literal-key combination in the real libdef — I was not able to reduce it to a minimal standalone snippet.

Triggering type (from the react-hook-form libdef)

declare export type DeepRequired<T> = T extends
    | BrowserNativeObject
    | Blob
    ? T
    : { [K in keyof T]-?: NonNullable<DeepRequired<T[K]>> };

// ... used downstream as:
declare export type FieldErrors<T: FieldValues = FieldValues> = Partial<
    FieldValues extends IsAny<FieldValues>
        ? any
        : FieldErrorsImpl<DeepRequired<T>>,
> & { root?: ({ [key: string]: GlobalError } & GlobalError) };

Checking any module that instantiates FieldErrors<SomeForm> (i.e. uses useForm<SomeForm>()) where SomeForm has primitive fields triggers the panic.

Expected behavior

Either accept the type or emit a normal type error — never panic. (0.318.0 / OCaml checks the entire codebase using this libdef with No errors!.)

Actual behavior (0.321.0)

The server crashes on startup / full check:

thread '<unnamed>' panicked at crates/flow_typing_flow_common/src/flow_js_utils.rs:1297:21:
Non speculating: Speculative(SpeculativeError(EInvalidObjectKit(EInvalidObjectKitData {
  reason: VirtualReasonInner { desc: RString,
    loc: ... LibFile("react-hook-form_v7.x.x.js") line: 255 ...,   // the `{ [K in keyof T]-?: ... }` mapped type in DeepRequired
    def_loc_opt: Some(... SourceFile("<a form component>") ...) },  // the concrete form field
  reason_op: VirtualReasonInner { desc: RObjectType, loc: ... line 255 ... },
  use_op: Op(TypeApplication {
    type_: ... RTypeApp(RType(Name("DeepRequired"))) ... line 276 ...  // FieldErrorsImpl<DeepRequired<T>>
  })
})))

Unhandled exception: Non speculating: Speculative(SpeculativeError(EInvalidObjectKit(...)))
Could not start Flow server!   (exit code 78)

desc varies with the offending field type (RString, RBoolean, …), confirming the trigger is the recursion reaching a primitive.

Workaround

Bottoming out the recursion on non-object types stops the panic:

declare export type DeepRequired<T> = T extends
    | BrowserNativeObject
    | Blob
    ? T
    : T extends { ... }
      ? { [K in keyof T]-?: NonNullable<DeepRequired<T[K]>> }
      : T;