[SebTardif]

What does this PR do?

A decorated private member is taken out of the class. A plain write, this.#p = value, was already lowered to __privateSet. Compound assignment (+=, &&=, ||=, ??=), ++ and --, destructuring, and for (this.#p of/in ...) were left as a call, or as a #p name outside the class. Loading the file then throws SyntaxError: Left side of assignment is not a reference, or Cannot reference undeclared private names.

Those forms now lower to __privateWrapper(object, member, setter, getter)._. That property is a real assignment target: the getter reads the member and the setter writes it. A direct this.#p = value stays __privateSet. When = is a destructuring default ({ 0: this.#p = 1 }), the left side stays a reference, because a call cannot be a destructuring target. Only private names that the decorator lowering extracted are rewritten, so an undecorated #u in the same class still behaves as a private field.

A decorated private method reads back the method after the brand check. Passing the method itself as the getter would call it. A write still throws, because the storage is a WeakSet and has no setter.

Field initializers still go through __privateAdd, so an accessor decorator's setter does not run for the initializer. It runs for later assignments.

The runtime transpiler cache version is now 45, so output cached before this lowering is not reused.

This is the bug reported in oven-sh#47043. oven-sh#42651 is an open change for the same bug that expands this.#p += 1 into a get plus a set, and uses the wrapper only for other receivers.

How did you verify your code works?

USE_SYSTEM_BUN=1 bun test test/bundler/transpiler/private-decorator-assign.test.ts on Bun 1.4.4 fails with SyntaxError: Left side of assignment is not a reference at this.#p += 1.

bun run build:release test test/bundler/transpiler/private-decorator-assign.test.ts passes. The fixture is a .js file. The same source in a .ts file does not hit this bug, which is why the existing decorator suite did not catch it. The run covers plain assignment, compound and logical assignment (including short-circuit), prefix and postfix update, object and array destructuring, rest, defaults, for-of, for-in, a wrapping accessor, an undecorated private next to a decorated one, and a decorated private method.

bun run build:release test test/regression/issue/cyclic-imports-async-bundler.test.js passes. __privateWrapper is the last export in runtime.js, so the part indexes of __esm and __promiseAll do not move and that snapshot's debugId stays the same.

cargo check -p bun_ast -p bun_js_parser --offline --tests passes. The sorted runtime-import unit test does not link under a raw cargo test here, because that link does not pull in mimalloc.

bun bd cannot run on this machine. scripts/build/deps/webkit-artifacts.json has no macOS arm64 debug or asan WebKit tarball, so the debug build stops before compile.