Restore the "double experiment registration" check and make it condit…

…ional on the ALLOW_EXPERIMENT_REREGISTRATION feature flag

package.json

@@ -19,7 +19,8 @@
 		"npm": ">=6.9.0 <7"
 	},
 	"config": {
-		"IS_GUTENBERG_PLUGIN": true
+		"IS_GUTENBERG_PLUGIN": true,
+		"ALLOW_EXPERIMENT_REREGISTRATION": true
 	},
 	"dependencies": {
 		"@wordpress/a11y": "file:packages/a11y",

packages/experiments/src/implementation.js

@@ -47,6 +47,15 @@ const registeredExperiments = [];
 const requiredConsent =
 	'I know using unstable features means my plugin or theme will inevitably break on the next WordPress release.';
 
+let allowReRegistration;
+// Use try/catch to force "false" if the environment variable is not explicitly
+// set to true (e.g. when building WordPress core).
+try {
+	allowReRegistration = process.env.ALLOW_EXPERIMENT_REREGISTRATION;
+} catch ( error ) {
+	allowReRegistration = false;
+}
+
 /**
  * Called by a @wordpress package wishing to opt-in to accessing or exposing
  * private experimental APIs.
@@ -68,19 +77,20 @@ export const __dangerousOptInToUnstableAPIsOnlyForCoreModules = (
 				'your product will inevitably break on one of the next WordPress releases.'
 		);
 	}
-	if ( ! process.env.IS_GUTENBERG_PLUGIN ) {
+	if (
+		! allowReRegistration &&
+		registeredExperiments.includes( moduleName )
+	) {
 		// This check doesn't play well with Story Books / Hot Module Reloading
 		// and isn't included in the Gutenberg plugin. It only matters in the
 		// WordPress core release.
-		if ( registeredExperiments.includes( moduleName ) ) {
-			throw new Error(
-				`You tried to opt-in to unstable APIs as module "${ moduleName }" which is already registered. ` +
-					'This feature is only for JavaScript modules shipped with WordPress core. ' +
-					'Please do not use it in plugins and themes as the unstable APIs will be removed ' +
-					'without a warning. If you ignore this error and depend on unstable features, ' +
-					'your product will inevitably break on one of the next WordPress releases.'
-			);
-		}
+		throw new Error(
+			`You tried to opt-in to unstable APIs as module "${ moduleName }" which is already registered. ` +
+				'This feature is only for JavaScript modules shipped with WordPress core. ' +
+				'Please do not use it in plugins and themes as the unstable APIs will be removed ' +
+				'without a warning. If you ignore this error and depend on unstable features, ' +
+				'your product will inevitably break on one of the next WordPress releases.'
+		);
 	}
 	if ( consent !== requiredConsent ) {
 		throw new Error(

test/unit/config/is-gutenberg-plugin.js

@@ -22,4 +22,11 @@ global.process.env = {
 	// eslint-disable-next-line @wordpress/is-gutenberg-plugin
 	IS_GUTENBERG_PLUGIN:
 		String( process.env.npm_package_config_IS_GUTENBERG_PLUGIN ) === 'true',
+	/**
+	 * Whether to allow the same experiment to be registered multiple times.
+	 * This is useful for development purposes, but should be set to false
+	 * during the unit tests to ensure the Gutenberg plugin can be cleanly
+	 * merged into WordPress core where this is false.
+	 */
+	ALLOW_EXPERIMENT_REREGISTRATION: false,
 };

tools/webpack/shared.js

@@ -66,6 +66,9 @@ const plugins = [
 		// Inject the `IS_GUTENBERG_PLUGIN` global, used for feature flagging.
 		'process.env.IS_GUTENBERG_PLUGIN':
 			process.env.npm_package_config_IS_GUTENBERG_PLUGIN,
+		// Inject the `ALLOW_EXPERIMENT_REREGISTRATION` global, used by @wordpress/experiments.
+		'process.env.ALLOW_EXPERIMENT_REREGISTRATION':
+			process.env.npm_package_config_ALLOW_EXPERIMENT_REREGISTRATION,
 	} ),
 	mode === 'production' && new ReadableJsAssetsWebpackPlugin(),
 ];