Simplify static + dynamic HTML generation

packages/next/build/webpack/loaders/next-serverless-loader/page-handler.ts

@@ -116,7 +116,7 @@ export function getPageHandler(ctx: ServerlessHandlerCtx) {
       previewProps: encodedPreviewProps,
       env: process.env,
       basePath,
-      requireStaticHTML: true, // Serverless target doesn't support streaming
+      supportsDynamicHTML: false, // Serverless target doesn't support streaming
       ..._renderOpts,
     }
     let _nextData = false

packages/next/export/index.ts

@@ -378,8 +378,8 @@ export default async function exportApp(
       domainLocales: i18n?.domains,
       trailingSlash: nextConfig.trailingSlash,
       disableOptimizedLoading: nextConfig.experimental.disableOptimizedLoading,
-      // TODO: We should support dynamic HTML too
-      requireStaticHTML: true,
+      // Exported pages do not currently support dynamic HTML.
+      supportsDynamicHTML: false,
       concurrentFeatures: nextConfig.experimental.concurrentFeatures,
     }
 

packages/next/server/next-server.ts

@@ -1250,13 +1250,12 @@ export default class Server {
       query: ParsedUrlQuery
     }
   ): Promise<void> {
-    // TODO: Determine when dynamic HTML is allowed
-    const requireStaticHTML = true
     const ctx = {
       ...partialContext,
       renderOpts: {
         ...this.renderOpts,
-        requireStaticHTML,
+        // TODO: Determine when dynamic HTML is allowed
+        supportsDynamicHTML: false,
       },
     } as const
     const payload = await fn(ctx)
@@ -1296,17 +1295,12 @@ export default class Server {
       ...partialContext,
       renderOpts: {
         ...this.renderOpts,
-        requireStaticHTML: true,
+        supportsDynamicHTML: false,
       },
     })
     if (payload === null) {
       return null
     }
-    if (payload.body.isDynamic()) {
-      throw new Error(
-        'invariant: expected a static result. This is a bug in Next.js'
-      )
-    }
     return payload.body.toUnchunkedString()
   }
 
@@ -1490,6 +1484,17 @@ export default class Server {
       delete query.amp
     }
 
+    if (opts.supportsDynamicHTML === true) {
+      // Disable dynamic HTML in cases that we know it won't be generated,
+      // so that we can continue generating a cache key when possible.
+      opts.supportsDynamicHTML =
+        !isSSG &&
+        !isLikeServerless &&
+        !query.amp &&
+        !this.minimalMode &&
+        typeof components.Document.getInitialProps !== 'function'
+    }
+
     const locale = query.__nextLocale as string
     const defaultLocale = isSSG
       ? this.nextConfig.i18n?.defaultLocale
@@ -1574,7 +1579,7 @@ export default class Server {
     }
 
     let ssgCacheKey =
-      isPreviewMode || !isSSG || this.minimalMode
+      isPreviewMode || !isSSG || this.minimalMode || opts.supportsDynamicHTML
         ? null // Preview mode bypasses the cache
         : `${locale ? `/${locale}` : ''}${
             (pathname === '/' || resolvedUrlPathname === '/') && locale

packages/next/server/render.tsx

@@ -194,7 +194,7 @@ export type RenderOptsPartial = {
   defaultLocale?: string
   domainLocales?: DomainLocale[]
   disableOptimizedLoading?: boolean
-  requireStaticHTML?: boolean
+  supportsDynamicHTML?: boolean
   concurrentFeatures?: boolean
   customServer?: boolean
 }
@@ -295,7 +295,7 @@ export async function renderToHTML(
     previewProps,
     basePath,
     devOnlyCacheBusterQueryString,
-    requireStaticHTML,
+    supportsDynamicHTML,
     concurrentFeatures,
   } = renderOpts
 
@@ -883,7 +883,20 @@ export async function renderToHTML(
     }
   }
 
-  const generateStaticHTML = requireStaticHTML || inAmpMode
+  /**
+   * Rules of Static & Dynamic HTML:
+   *
+   *    1.) We must generate static HTML unless the caller explicitly opts
+   *        in to dynamic HTML support.
+   *
+   *    2.) If dynamic HTML support is requested, we must honor that request
+   *        or throw an error. It is the sole responsibility of the caller to
+   *        ensure they aren't e.g. requesting dynamic HTML for an AMP page.
+   *
+   * These rules help ensure that other existing features like request caching,
+   * coalescing, and ISR continue working as intended.
+   */
+  const generateStaticHTML = supportsDynamicHTML !== true
   const renderToStream = (element: React.ReactElement) =>
     new Promise<Observable<string>>((resolve, reject) => {
       const stream = new PassThrough()