Function initializers as properties

[sidlamsal]

Fixes #59112

[Copilot]

Pull Request Overview

This PR updates the binder logic for expando initializers when they are function‐like, distinguishing between prototype properties and non‑prototype properties.

• When the expando is assigned to a prototype, only the Method flag is set; otherwise, both Method and Property flags are combined.

[sandersn]

This change needs tests. This probably changes tonnes of existing tests, so it's a matter of

1. $ hereby runtests-parallel
2. $ hereby baseline-accept

and then committing the updates in tests/baselines/reference

[sandersn]

This should just be Property, I'm pretty sure. They're a function property, but don't have useful this semantics, static vs instance distinction, etc.

[sandersn]

Since you don't need a custom includes you can reduce this code change down to

Suggested change

	if (isFunctionLikeDeclaration(getAssignedExpandoInitializer(declaration)!)) {
	if (isPrototypeProperty && isFunctionLikeDeclaration(getAssignedExpandoInitializer(declaration)!)) {

and the default case at the bottom of the function will apply.

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate