7.1 regression: declaration order of recursive generic interfaces changes assignability (TS2322) #64891

🔎 Search Terms

variance, circular variance, declaration order, mapped type as clause, key remapping, TS2322, 7.1 regression

🕗 Version & Regression Information

• Last good: 7.1.0-dev.20260805.1 (gitHead 12318e599d)
• First bad: 7.1.0-dev.20260806.1 (gitHead 86cc4767d4)
• That range is one commit: Order variance computation by associated type symbol typescript-go#4820, "Order variance computation by associated type symbol".
• Still failing on 7.1.0-beta and 7.1.0-dev.20261020.3.
• TypeScript 6.0.3 and 7.0.2 accept the code.

⏯ Playground Link

The tested 7.1 builds aren't available in the Playground: requesting 7.1.0-dev.20261020.3 reports that it hasn't been prepared, and Nightly currently loads 6.0.0-dev.20260416. The CLI repro is two files in an empty directory, with no package.json, run with tsc -p .:

tsconfig.json

{ "compilerOptions": { "noEmit": true, "strict": true }, "files": ["repro.ts"] }

In my runs the result did not change with strict off, or with module/moduleResolution set to nodenext or to esnext/bundler.

💻 Code

repro.ts

type M = { kind: "m" };
type Filtered<T> = { [K in keyof T as T[K] extends M ? K : never]: T[K] };
interface Config<T> { measure: keyof T }
interface Ref<N> { dataset: DS<Record<string, M>, N> }
interface DS<T, N> {
  a(config: Config<Filtered<T>>): void;
  b: Ref<N>;
}
declare const x: DS<{ total: M }, string>;
export const y: DS<Record<string, M>, string> = x;

🙁 Actual behavior

repro.ts(10,14): error TS2322: Type 'DS<{ total: M; }, string>' is not assignable to type 'DS<Record<string, M>, string>'.
  Property 'total' is missing in type 'Record<string, M>' but required in type '{ total: M; }'.

The reverse assignment (DS<Record<string, M>, string> to DS<{ total: M }, string>) also fails. This is consistent with T being treated as invariant. The diagnostic reports the missing total property on the type argument rather than an incompatibility in a or b. I haven't traced whether a structural comparison is attempted.

🙂 Expected behavior

No error, as in 6.0.3 and 7.0.2. The assignment is valid member by member. a is a method, so its parameter is checked bivariantly ({ measure: "total" } against { measure: string }). b is the same type on both sides. Replacing the final assignment with this member-by-member version succeeds on both tested 7.1 builds:

export const y: DS<Record<string, M>, string> = { a: x.a, b: x.b };

⏯ Additional information about the issue

The result depends on declaration order. Each of these changes, applied on its own to the repro above, makes the assignment pass on both 7.1.0-beta and 7.1.0-dev.20261020.3:

1. Declaring DS before Ref.
2. Adding this comparison before the failing assignment:

   declare const r: Ref<"a">;
   export const r2: Ref<string> = r;

3. Writing a's parameter as { measure: keyof Filtered<T> } instead of Config<Filtered<T>>.
4. Dropping the as clause from Filtered.
5. Removing N from DS, Ref and their instantiations, so that Ref is not generic.

DS.b doesn't mention T. Ref<N>.dataset always fixes DS's first argument to Record<string, M> and passes through only N. Even so, swapping the declarations changes whether the assignment succeeds.

Real-world impact: this is reduced from @hypequery/datasets 0.23.1. A dataset created with named total and count measures assigns to the exported AnyDatasetInstance on 6.0.3 and 7.0.2, but fails with TS2322 on the tested 7.1 builds. DatasetInstance.metric() takes BaseMetricConfig<BaseMeasures<TMeasures>>, where BaseMeasures uses key remapping. Its returned references carry the concrete dataset type. Separately, DerivedMetricConfig.uses holds BaseMetricRef<TDatasetName> references whose default dataset type is the wide one.