[lexical] Fix: make the extension Flow types usable (defineExtension was uncallable)

[potatowagon]

The hand-maintained Flow declarations mistranslate the unique symbol
brands that carry an extension's Config/Output/Init types, which makes
defineExtension — and therefore every extension-built editor — impossible
to call from a Flow codebase.

packages/lexical/src/extension-core/internal.ts declares three optional
properties keyed by unique symbol:

export declare const configTypeSymbol: unique symbol;
export interface LexicalExtensionInternal<Config, Output, Init> {
  readonly [configTypeSymbol]?: Config;
  readonly [outputTypeSymbol]?: Output;
  readonly [initTypeSymbol]?: Init;
}

The Flow mirror drops both halves of that: the symbols become plain
symbol, and the properties become required.

declare export const configTypeSymbol: symbol;
interface LexicalExtensionInternalConfig<Config> {
  [typeof configTypeSymbol]: Config;
}

Flow has no unique symbol. A computed key whose type is symbol is an
indexer over all symbol keys, not a single named property, so every other
property of an extension has to conform to the indexer's value type. The
result is that no object literal can satisfy LexicalExtension:

Cannot call `defineExtension` with object literal bound to `extension`
because property `name` is missing in `LexicalExtensionInternal` but
exists in object literal. Any property that does not exist in
`LexicalExtensionInternal` must be compatible with its indexer
`typeof initTypeSymbol`.

Declaring all three also trips Multiple indexers are not supported.

There is no caller-side workaround: explicit type arguments do not help,
because the object literal itself is rejected.

This replaces the symbol-keyed brands with named optional read-only
properties, which reproduce the TypeScript semantics that matter — the three
types are carried, callers never supply the properties, and
LexicalExtension{Config,Output,Init} still extract them. The symbols stay
exported so the .flow surface keeps matching the generated .d.ts that
pnpm lint-flow compares against.

It also gives defineExtension's type parameters defaults. Config,
Output and Init are reachable only through the optional config,
build and init members, so an extension declaring none of them leaves
them unconstrained; TypeScript infers silently there, Flow reports
underconstrained-implicit-instantiation. This surfaced only once the
indexer error was gone.

Finally, this adds packages/lexical/flow/Lexical.flowtest.js, a type-only
consumer of the public extension API. pnpm lint-flow compares export
names between the .flow and .d.ts files, and pnpm flow checks the
declarations for internal consistency, but nothing in the repo ever calls
the declared API — which is why a completely uncallable defineExtension
passed CI. The new file closes that gap. It is type-only, is never imported
at runtime, and flow/ is not in the package's published files.

Test Plan:
Verified against the identical declarations as synced into Meta's www
monorepo, using www's Flow (0.331) — the same Lexical.js.flow content, so
the reproduction and the fix both apply to this file.

Before: a file calling defineExtension produces the indexer errors quoted
above. Four spellings were tried — bare object literal, explicit type
arguments, with and without config — and all four fail.

After (this change, applied to that copy): flow status reports
No errors! for a consumer that

• calls defineExtension with no config/build/init,
• calls it with a config and a build returning an output,
• and reads both back through LexicalExtensionConfig<typeof ext> and
  LexicalExtensionOutput<typeof ext>.
  That consumer is what Lexical.flowtest.js contains.

Not verified with this repo's own pnpm flow (flow-bin 0.321 could not be
installed in the sandbox used), so please let CI confirm the new test file
parses cleanly under the pinned version.

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Project	Deployment	Actions	Updated
lexical	Ready	Preview	Sep 8, 2026 7:27am UTC
lexical-playground	Ready	Preview	Sep 8, 2026 7:27am UTC