Enable allowOptionalDependencies by default (#1775)

[robhogan]

Summary:

Flip the default value of the transformer.allowOptionalDependencies Metro config option from false to true in metro-config's own defaults.

This allows for patterns like:

let dep = null;
try {
  // If not installed, allowOptionalDependencies ===
  //   true => throw at runtime, handled by catch
  //   false => throw at build time
  dep = require('maybe-installed-dependency');
} catch {
  dep = require('./my-fallback');
}

Which comes up in popular packages, eg lru-cache.

Note that this is already the default in:

• react-native/metro-config: https://github.com/react/react-native/blob/a632f9efe24bac8b3a113c78469948f55bde0f5d/packages/metro-config/src/index.flow.js#L86
• And expo/metro-config: https://github.com/expo/expo/blob/ab042edb8228e3873bdbd8e04b7c61cd3e00372e/packages/%40expo/metro-config/src/ExpoMetroConfig.ts#L423

There were previously known problems with allowOptionalDependencies and the behaviour of subsequently declared dependencies, and they were also broken under HMR if their existence changed. These were both fixed a year ago in #1522.

The heuristic was also quite limited, for example a try { require() } was considered optional but an import().catch() was not. That was fixed in #1697.

I can't think of any good reason not to enable this and fix the misalignment with RN/Expo.

With this enabled, dependencies referenced inside try/catch blocks (and dynamic import()s with rejection handlers) are treated as optional by default: an unresolvable optional dependency no longer fails the build, matching the common expectation for guarded requires. Projects can still opt out by explicitly setting allowOptionalDependencies: false.

Since this change doesn't alter observable runtime behaviour (it allows bundles to build that would otherwise fail to build), I'm inclined to regard it as non-breaking.

 - **[Feature]**: `allowOptionalDependencies` defaults to `true` to align with RN and Expo

Reviewed By: huntie

Differential Revision: D111577052

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D111577052.

[meta-codesync]

This pull request has been merged in 948f730.