[BUGFIX] Keep generic classes generic through extend()

[NullVoxPopuli]

Note

Investigated with Claude (via Claude Code); corrections welcome.

Component.extend(Mixin) now keeps Component<S> generic, so a class that extends it can still take a signature. Today it can't. This matters most in JS, where TypeScript 7 reports TS2508 for this pattern and the component loses its signature.

/** @extends {Component<Signature>} */
export default class Foo extends Component.extend(Evented) {}

The change

• Before: Readonly<Statics> & EmberClassConstructor<Instance> & MergeArray<M>. EmberClassConstructor<Instance> is new (owner?) => Instance, with Instance already fixed to Component<unknown>.
• After: Statics & MergeArray<M>. Statics is the this type, so its generic construct signatures carry through.

Behavior change

Statics on the result are no longer Readonly. No type test relied on that. Adding Readonly<Statics> back to the intersection would not restore it: an intersection property is readonly only if every member is.

Why now

• TS 5.x let a JSDoc @extends replace the base type no matter what extend() returned.
• TypeScript 7 instead applies the tag's type arguments to the base constructor (fix(64058): fix reparse jsdoc @extends type arguments for call expressions microsoft/TypeScript#64072, for TS7: @extends is ignored when the heritage is a call expression (Base.extend()) microsoft/TypeScript#64058). With a constructor that isn't generic, that's TS2508: No base constructor has the specified number of type arguments.
• This hits classic components in .gjs with ember-content-mapper (JSDoc @extends signature is lost for classic components with expression heritage (Component.extend(Mixin)) NullVoxPopuli/ember-content-mapper#12).

Testing

• A new type test in type-tests/@ember/object-test/extend.ts covers class StringBox extends Box.extend({})<string> (through a const). It fails on main with the same TS2508 and passes with this change.
• type-check:internals, type-check:internals-stable-decorators, build:types, type-check:types and type-check:types-stable-decorators all pass (TS 5.9.3).
• type-check:types and type-check:types-stable-decorators also pass on TS 5.2.2 and 7.0.2, the two ends of the types-range CI matrix.
• In ember-content-mapper's upstream-differences fixture, the classic Component.extend(Evented) component reports no errors on TS 7.1.0-dev.20261015.1 with these types. Today it reports TS2508 and TS2769.

🤖 Generated with Claude Code

[github-actions]

📊 Size report

Tarball size — 1.3 MB → 1.3 MB

dist/dev   No changes

File	Before (Size / Brotli)	After (Size / Brotli)
./packages/shared-chunks/api-{hash}.js	5.9 kB / 1.9 kB	358%↑27 kB / 222%↑6.1 kB
./packages/shared-chunks/index-{hash}.js	57.1 kB / 14.1 kB	10%↑62.6 kB / -10.9%↓12.5 kB
Total (Includes all files)	2.2 MB / 519.9 kB	2.2 MB / 519.9 kB

---

dist/prod   No changes

File	Before (Size / Brotli)	After (Size / Brotli)
Total (Includes all files)	2 MB / 476.6 kB	2 MB / 476.6 kB

---

smoke-tests/v2-app-template/dist   No changes

File	Before (Size / Brotli)	After (Size / Brotli)
Total (Includes all files)	359.3 kB / 97.1 kB	359.3 kB / 97.1 kB

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smoke-tests/v2-app-hello-world-template/dist   No changes

File	Before (Size / Brotli)	After (Size / Brotli)
Total (Includes all files)	128.9 kB / 35.3 kB	128.9 kB / 35.3 kB

_{🤖 This report was automatically generated by wyvox/pkg-size}