Fix --use-pnp for Yarn 2 (facebook#8460)

Author: nickserv

packages/create-react-app/createReactApp.js

@@ -261,17 +261,26 @@ function createApp(
     }
   } else if (usePnp) {
     const yarnInfo = checkYarnVersion();
-    if (!yarnInfo.hasMinYarnPnp) {
-      if (yarnInfo.yarnVersion) {
+    if (yarnInfo.yarnVersion) {
+      if (!yarnInfo.hasMinYarnPnp) {
         console.log(
           chalk.yellow(
             `You are using Yarn ${yarnInfo.yarnVersion} together with the --use-pnp flag, but Plug'n'Play is only supported starting from the 1.12 release.\n\n` +
               `Please update to Yarn 1.12 or higher for a better, fully supported experience.\n`
           )
         );
+        // 1.11 had an issue with webpack-dev-middleware, so better not use PnP with it (never reached stable, but still)
+        usePnp = false;
+      }
+      if (!yarnInfo.hasMaxYarnPnp) {
+        console.log(
+          chalk.yellow(
+            'The --use-pnp flag is no longer necessary with yarn 2 and will be deprecated and removed in a future release.\n'
+          )
+        );
+        // 2 supports PnP by default and breaks when trying to use the flag
+        usePnp = false;
       }
-      // 1.11 had an issue with webpack-dev-middleware, so better not use PnP with it (never reached stable, but still)
-      usePnp = false;
     }
   }
 
@@ -775,14 +784,17 @@ function checkNpmVersion() {
 
 function checkYarnVersion() {
   const minYarnPnp = '1.12.0';
+  const maxYarnPnp = '2.0.0';
   let hasMinYarnPnp = false;
+  let hasMaxYarnPnp = false;
   let yarnVersion = null;
   try {
     yarnVersion = execSync('yarnpkg --version')
       .toString()
       .trim();
     if (semver.valid(yarnVersion)) {
       hasMinYarnPnp = semver.gte(yarnVersion, minYarnPnp);
+      hasMaxYarnPnp = semver.lt(yarnVersion, maxYarnPnp);
     } else {
       // Handle non-semver compliant yarn version strings, which yarn currently
       // uses for nightly builds. The regex truncates anything after the first
@@ -791,13 +803,15 @@ function checkYarnVersion() {
       if (trimmedYarnVersionMatch) {
         const trimmedYarnVersion = trimmedYarnVersionMatch.pop();
         hasMinYarnPnp = semver.gte(trimmedYarnVersion, minYarnPnp);
+        hasMaxYarnPnp = semver.lt(trimmedYarnVersion, maxYarnPnp);
       }
     }
   } catch (err) {
     // ignore
   }
   return {
     hasMinYarnPnp: hasMinYarnPnp,
+    hasMaxYarnPnp: hasMaxYarnPnp,
     yarnVersion: yarnVersion,
   };
 }