Google feedback on TS 6.0-beta

### Acknowledgement

- [x] I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

### Comment

This GitHub issue contains feedback on the TS 6.0-beta release from the team
that is responsible for keeping Google's internal software working with the
latest version of TypeScript.

## Executive summary

- We do not expect to have significant difficulty in upgrading Google to TS
6.0.
- Some changes to our TypeScript code are required to fix compilation.
- Most of the new errors are clearly related to announced changes.
- Detail sections below explain the changes we expect to make to unblock
the upgrade.
- We expect some deprecations to cause difficulty upgrading to TS 7.0.
- We plan to provide feedback about this at a later date.

## Impact summary

| Change description | Announced | Libraries affected |
| ------------------------------------- | --------: | -----------------: |
| `strict` defaults to `true` | Yes | ~0.1% |
| Legacy `module` syntax for namespaces deprecated | Yes | ~0.02% |

The **Announced** column indicates whether we were able to connect the observed
change with a section in the [TS6.0-beta announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-6-0-beta/).

The following sections give more detailed explanations of the changes listed
above.

## Announced changes for TS 6.0

### `strict` now defaults to `true`

We support this change.

We will set `strict` to `false` until we clean up newly revealed errors in our
codebase. We have not previously enabled `strictBuiltinIteratorReturn` and
`alwaysStrict`.

### `lib.d.ts` changes

We support these changes.

As part of this migration, for impacted `.d.ts` files, we will update
conflicting typings where necessary. For impacted `.ts` files, we will add
`// @ts-ignore` suppressions to silence pre-existing errors with a note advising
code authors to revisit the code and fix these genuine problems.

### `module` syntax for namespaces is deprecated

We support this change.

We will update usages of `module` that declare namespaces to `namespace`.

### `"ignoreDeprecations": "6.0"`

We will set this in order to roll out TS 6.0 as soon as possible. We plan to
provide additional feedback about the impact of these deprecations on TS 7.0 at
a later date.

## Deprecations affecting TS 7.0

The following deprecations do not affect our adoption of TS 6.0 but will need to
be addressed before we adopt TS 7.0.

### `es5` and `--downlevelIteration` deprecation

We currently use ES5 output for a small number of legacy development purposes
and are exploring using another transpiler to continue producing it in the
future.

### `--baseUrl` deprecation

We have resolved this issue by prepending the previous `baseUrl` value to our
path mapping.

### `--esModuleInterop false` and `--allowSyntheticDefaultImports false` deprecations

We have code patterns that rely on setting these options to true and will be
evaluating how to proceed.

### Other changes

We support the following changes but have nothing else to add:

- `es2025` option for `target` and `lib`
- New types for `Temporal`
- New types for "upsert" methods
- `Regexp.escape`

The following changes are largely irrelevant in our codebase and we don't have
anything to add about them:

- Subpath imports starting with `#/`
- Combining `--moduleResolution` bundler with `--module commonjs`
- `rootDir` now defaults to `.`
- `types` now defaults to `[]`
- Other default changes
- Other deprecations

## Unannounced changes

We were not able to associate any new breakages with specific unannounced
changes. If we find any, we will update this.

Read the TS 6.0-beta announcement and the issue sections covering strict defaults, lib.d.ts, namespace syntax, and deprecations. No source file, test, entry point, or specific compiler change is named; a concrete reproducible breakage or requested change would be needed to define done.

Written by the indexing model from the issue text.