restoring an hbs fallback case that was actually needed

Author: ef4

packages/vite/src/hbs.ts

@@ -7,6 +7,7 @@ import {
   needsSyntheticComponentJS,
   isInComponents,
   templateOnlyComponentSource,
+  syntheticJStoHBS,
 } from '@embroider/core';
 
 const resolverLoader = new ResolverLoader(process.cwd());
@@ -31,18 +32,46 @@ export function hbs(): Plugin {
         skipSelf: true,
       });
 
-      if (resolution) {
-        let syntheticId = needsSyntheticComponentJS(source, resolution.id);
-        if (syntheticId && isInComponents(resolution.id, resolverLoader.resolver.packageCache)) {
-          return {
-            id: syntheticId,
-            meta: {
-              'rollup-hbs-plugin': {
-                type: 'template-only-component-js',
+      if (!resolution) {
+        // vite already has extension search fallback for extensionless imports.
+        // This is different, it covers an explicit .js import fallback to the
+        // corresponding hbs.
+        let hbsSource = syntheticJStoHBS(source);
+        if (hbsSource) {
+          resolution = await this.resolve(hbsSource, importer, {
+            skipSelf: true,
+            custom: {
+              embroider: {
+                // we don't want to recurse into the whole embroider compatbility
+                // resolver here. It has presumably already steered our request to the
+                // correct place. All we want to do is slightly modify the request we
+                // were given (changing the extension) and check if that would resolve
+                // instead.
+                //
+                // Currently this guard is only actually exercised in rollup, not in
+                // vite, due to https://github.com/vitejs/vite/issues/13852
+                enableCustomResolver: false,
+                isExtensionSearch: true,
               },
             },
-          };
+          });
         }
+
+        if (!resolution) {
+          return null;
+        }
+      }
+
+      let syntheticId = needsSyntheticComponentJS(source, resolution.id);
+      if (syntheticId && isInComponents(resolution.id, resolverLoader.resolver.packageCache)) {
+        return {
+          id: syntheticId,
+          meta: {
+            'rollup-hbs-plugin': {
+              type: 'template-only-component-js',
+            },
+          },
+        };
       }
 
       return resolution;