[fs-detectors] Exclude the middleware builder if it's a Next.js app (#8239)

packages/fs-detectors/src/detect-builders.ts

@@ -99,7 +99,7 @@ export async function detectBuilders(
   const errors: ErrorResponse[] = [];
   const warnings: ErrorResponse[] = [];
 
-  const apiBuilders: Builder[] = [];
+  let apiBuilders: Builder[] = [];
   let frontendBuilder: Builder | null = null;
 
   const functionError = validateFunctions(options);
@@ -305,6 +305,24 @@ export async function detectBuilders(
     };
   }
 
+  // Exclude the middleware builder for Next.js apps since @vercel/next
+  // will build middlewares.
+  //
+  // While maybeGetApiBuilder() excludes the middleware builder, however,
+  // we need to check if it's a Next.js app here again for the case where
+  // `projectSettings.framework == null`.
+  if (
+    framework === null &&
+    frontendBuilder?.use === '@vercel/next' &&
+    apiBuilders.length > 0
+  ) {
+    apiBuilders = apiBuilders.filter(builder => {
+      const isMiddlewareBuilder =
+        builder.use === '@vercel/node' && builder.config?.middleware;
+      return !isMiddlewareBuilder;
+    });
+  }
+
   const builders: Builder[] = [];
 
   if (apiBuilders.length) {

packages/fs-detectors/test/unit.builds-and-routes-detector.test.ts

@@ -2281,6 +2281,31 @@ describe('Test `detectBuilders` with `featHandleMiss=true`', () => {
       },
     ]);
   });
+
+  it('should not add middleware builder when building "nextjs"', async () => {
+    const files = ['package.json', 'pages/index.ts', 'middleware.ts'];
+    const pkg = {
+      scripts: { build: 'next build' },
+      dependencies: { next: '12.2.0' },
+    };
+    const projectSettings = {
+      framework: null, // "Other" framework
+      createdAt: Date.parse('2020-02-01'),
+    };
+    const { builders } = await detectBuilders(files, pkg, {
+      projectSettings,
+      featHandleMiss,
+    });
+    expect(builders).toEqual([
+      {
+        use: '@vercel/next',
+        src: 'package.json',
+        config: {
+          zeroConfig: true,
+        },
+      },
+    ]);
+  });
 });
 
 it('Test `detectRoutes`', async () => {