Regression in 7.1 (#64553): R inferred as unknown through a generic type alias call signature with a nested same-type argument #64757

🔎 Search Terms

inference unknown, generic type alias call signature, generator, invokeOnce, isDeeplyNestedType, #64553, cache inferences type arguments, regression 7.1

🕗 Version & Regression Information

• This changed between 7.1.0-dev.20260929.1 and 7.1.0-dev.20261001.1.
• It is still present in 7.1.0-dev.20261012.1 (next).
• 7.0.2 and 5.9.3 infer the expected type.
• The tsc-rs maintainers bisected the Go build to c8e9b7d, Cache inferences made from type arguments #64553 ("Cache inferences made from type arguments"). The parent commit gives the expected type. Details: 0.4.0 regression: Effect HttpRouter.middleware(...).layer types as string (requires inferred unknown); 0.3.0 and tsc 7.1.0-dev accept pingdotgg/ts-rust#79 (comment)

⏯ Playground Link

No playground link. The code below is self-contained, with no dependencies.

💻 Code

interface Effect<out A, out R = never> {
  readonly _A: () => A;
  readonly _R: () => R;
  [Symbol.iterator](): Iterator<Effect<A, R>, A>;
}
declare function succeed<A>(value: A): Effect<A>;
declare function gen<Eff extends Effect<any, any>, AEff>(
  body: () => Generator<Eff, AEff, never>,
): () => Effect<AEff, Eff extends Effect<infer _A, infer R> ? R : never>;

interface Req { readonly url: string }
declare const req: Effect<Req, Req>;

type Make<P> = <R>(m: Effect<() => Effect<string, R>>) => [R];
declare const make: Make<unknown>;

const r = make(succeed(gen(function* () {
  yield* req;
  return "";
})));
const check: [1] = r; // error message shows what R was inferred as

tsconfig.json: { "compilerOptions": { "target": "ES2023", "module": "NodeNext", "strict": true, "noEmit": true } }

🙁 Actual behavior

R is inferred as unknown, so r is [unknown].

🙂 Expected behavior

R is inferred as Req, so r is [Req], as in 5.9.3, 7.0.2 and 7.1.0-dev.20260929.1.

Additional information about the issue

The tsc-rs maintainers traced it to the depth-2 nesting check in invokeOnce (isDeeplyNestedType(..., 2), about line 363 of inference.go). #64553 now sends the Effect/Effect pair through it. For Effect<() => Effect<A, R>>, the check treats the inner pair as a cycle and infers nothing for the inner R. It only happens when the signature comes from an instantiated generic type alias (Make<P> above). Calling a plain generic function with the same shape infers Req.

Real-world impact: this is the shape of Effect 4's HttpRouter.middleware<...>()(...), whose signature is the generic alias middleware.Make<Provides, Handles>. A middleware written with Effect.fn or Effect.gen that reads the request gets R = unknown, and .layer resolves to the string type "Need to .combine(middleware) that satisfy the missing request dependencies". In one Effect app this cascades into 145 errors.