7.1 regression: circular type alias accepts an assignment it should reject (opposite of #64891)

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@jasleenkaur-qed42 がすでに取り組んでいます。

2026年10月23日 から。

  • #64935 @jasleenkaur-qed42 による — オープン
  • #65030 @1-DARK による — オープン

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調査の方向性

Start by running the supplied TypeScript repro with strict mode and compare its result across the noted compiler versions; the report identifies commit microsoft/typescript-go#4820 and a related case in #64891. Trace how circular aliases with two type parameters are checked, including the reported declaration-order and check-order effects. Done when the repro reports TS2322 and the related case still behaves as expected.

索引モデルが issue の本文から書いたものです。

説明

Bug
Search Terms

circular type alias variance, missing TS2322, unrelated type arguments assignable, 7.1 regression, #64891

Version & Regression Information
  • This changed between versions 7.0.2 and 7.1.0-dev.20261022.1
  • This changed in commit or PR microsoft/typescript-go#4820

I bisected it with the nightlies: 7.1.0-dev.20260805.1 (built from 12318e599) reports the error, and 7.1.0-dev.20260806.1 (built from 86cc4767d, the #4820 merge) doesn't.

Playground Link

No response

Code
type Params<T, K> = { config: Config<T, K> };
type Handler<T, K> = (params: Params<T, K>) => void;
type Config<T, K> = { read: (item: T) => void; handler: Handler<T, K> };

declare const a: Handler<string, string>;
export const b: Handler<number, string> = a; // should be TS2322

{ "compilerOptions": { "strict": true, "noEmit": true, "types": [] }, "files": ["a.ts"] }
Actual behavior

No errors. A handler that reads strings is accepted where one that reads numbers is expected.

It's not just close types either: Handler<boolean, { x: 1 }> is accepted too.

Expected behavior

TS2322 on the last line, same as 6.0.3 and 7.0.2.

Additional information about the issue

Hi there!

Some context on how I found this: I've been testing out the latest 7.1 nightlies and I'm super excited about them, mainly for the memory and CPU speedups. We have a very large TypeScript monorepo in production, so those gains make a real difference for us.

While digging into #64891 I found the opposite case, and I think it's the other half of the same bug. That issue is about 7.1 rejecting code it should accept. This one is the opposite, it accepts code it should reject, which is a bit scarier because nothing tells you it happened.

Here's what I see with tsc -p . (same result on every run):

Build This repro #64891 repro
6.0.3 TS2322 (correct) no errors (correct)
7.0.2 TS2322 (correct) no errors (correct)
7.1.0-dev.20261022.1 no errors (wrong) TS2322 (wrong)
#64893 (6e72aa76c, built locally) TS2322 (correct) no errors (correct)

A few other things I noticed while narrowing it down:

  • It needs two type parameters. With only T, every build reports the error.

  • If I move Handler above Params, the nightly reports the error again.

  • #64893 fixes both repros, but the result still depends on what gets checked first. If Params is compared before Handler, every build I tried misses the Handler error, #64893 included:

    declare const p: Params<string, string>;
export const q: Params<number, string> = p; // TS2322 everywhere
declare const a: Handler<string, string>;
export const b: Handler<number, string> = a; // missed everywhere

    So I'm not sure #64893 is the full fix, but it does get things back to how 7.0.2 behaves.

Thanks for all the work on 7.x, it's been great so far!

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