404 routing for static export without exportPathMap (#5036)

Resolves #5035 

Thanks a billion to @timneutkens for holding my hand

server/export.js

@@ -31,6 +31,12 @@ export default async function (dir, options, configuration) {
     if (page === '/_document' || page === '/_app') {
       continue
     }
+
+    if (page === '/_error') {
+      defaultPathMap['404'] = { page }
+      continue
+    }
+
     defaultPathMap[page] = { page }
   }
 

test/integration/static/test/ssr.js

@@ -48,5 +48,10 @@ export default function (context) {
       const $ = cheerio.load(html)
       expect($('img').attr('src')).toBe('/static/myimage.png')
     })
+
+    it('should render _error on 404', async () => {
+      const html = await renderViaHTTP(context.port, '/404')
+      expect(html).toMatch(/404/)
+    })
   })
 }