From 2ea2c81f52b74b96f65ba4e2a08e404d31d5205b Mon Sep 17 00:00:00 2001
From: Thibaut SABOT <9283289+thibautsabot@users.noreply.github.com>
Date: Sat, 5 Feb 2022 21:16:25 +0100
Subject: [PATCH] Don't use yarn if a package-lock.json file is found (#31926)

Fixes https://github.com/vercel/next.js/issues/31755
---
 packages/next/lib/helpers/should-use-yarn.ts | 14 +++++++++++---
 packages/next/lib/install-dependencies.ts    |  2 +-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/packages/next/lib/helpers/should-use-yarn.ts b/packages/next/lib/helpers/should-use-yarn.ts
index c305361517f0b2e..27c15b1dbac7da2 100644
--- a/packages/next/lib/helpers/should-use-yarn.ts
+++ b/packages/next/lib/helpers/should-use-yarn.ts
@@ -1,13 +1,21 @@
 import { execSync } from 'child_process'
+import fs from 'fs'
+import path from 'path'
 
-export function shouldUseYarn(): boolean {
+export function shouldUseYarn(baseDir: string): boolean {
   try {
     const userAgent = process.env.npm_config_user_agent
     if (userAgent) {
       return Boolean(userAgent && userAgent.startsWith('yarn'))
+    } else {
+      if (fs.existsSync(path.join(baseDir, 'yarn.lock'))) {
+        return true
+      } else if (fs.existsSync(path.join(baseDir, 'package-lock.json'))) {
+        return false
+      }
+      execSync('yarnpkg --version', { stdio: 'ignore' })
+      return true
     }
-    execSync('yarnpkg --version', { stdio: 'ignore' })
-    return true
   } catch (e) {
     return false
   }
diff --git a/packages/next/lib/install-dependencies.ts b/packages/next/lib/install-dependencies.ts
index 4c0399b0068890e..77987f0ddf7bd90 100644
--- a/packages/next/lib/install-dependencies.ts
+++ b/packages/next/lib/install-dependencies.ts
@@ -15,7 +15,7 @@ export async function installDependencies(
   deps: any,
   dev: boolean = false
 ) {
-  const useYarn = shouldUseYarn()
+  const useYarn = shouldUseYarn(baseDir)
   const isOnline = !useYarn || (await getOnline())
 
   if (deps.length) {