From 5f7ea0d08c202f6c30047ef3e2df5bb25cb20236 Mon Sep 17 00:00:00 2001
From: Thibaut Sabot <thibaut.sabot@adevinta.com>
Date: Mon, 29 Nov 2021 15:55:57 +0100
Subject: [PATCH 1/2] Don't use yarn if a package-lock.json file is found

---
 packages/next/lib/helpers/should-use-yarn.ts | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/packages/next/lib/helpers/should-use-yarn.ts b/packages/next/lib/helpers/should-use-yarn.ts
index c305361517f0b2e..5f5018c0dbc5f77 100644
--- a/packages/next/lib/helpers/should-use-yarn.ts
+++ b/packages/next/lib/helpers/should-use-yarn.ts
@@ -1,13 +1,20 @@
 import { execSync } from 'child_process'
+import fs from 'fs'
 
 export function shouldUseYarn(): boolean {
   try {
     const userAgent = process.env.npm_config_user_agent
     if (userAgent) {
       return Boolean(userAgent && userAgent.startsWith('yarn'))
+    } else {
+      if (fs.existsSync('yarn.lock')) {
+        return true
+      } else if (fs.existsSync('package-lock.json')) {
+        return false
+      }
+      execSync('yarnpkg --version', { stdio: 'ignore' })
+      return true
     }
-    execSync('yarnpkg --version', { stdio: 'ignore' })
-    return true
   } catch (e) {
     return false
   }

From 6472406d395f85950002acfd9eb0c0cc057bda39 Mon Sep 17 00:00:00 2001
From: Thibaut Sabot <thibaut.sabot@adevinta.com>
Date: Mon, 29 Nov 2021 16:06:34 +0100
Subject: [PATCH 2/2] use baseDir to find lock files

---
 packages/next/lib/helpers/should-use-yarn.ts | 7 ++++---
 packages/next/lib/install-dependencies.ts    | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/packages/next/lib/helpers/should-use-yarn.ts b/packages/next/lib/helpers/should-use-yarn.ts
index 5f5018c0dbc5f77..27c15b1dbac7da2 100644
--- a/packages/next/lib/helpers/should-use-yarn.ts
+++ b/packages/next/lib/helpers/should-use-yarn.ts
@@ -1,15 +1,16 @@
 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('yarn.lock')) {
+      if (fs.existsSync(path.join(baseDir, 'yarn.lock'))) {
         return true
-      } else if (fs.existsSync('package-lock.json')) {
+      } else if (fs.existsSync(path.join(baseDir, 'package-lock.json'))) {
         return false
       }
       execSync('yarnpkg --version', { stdio: 'ignore' })
diff --git a/packages/next/lib/install-dependencies.ts b/packages/next/lib/install-dependencies.ts
index 79184113e14f87b..a27133b472c6db6 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) {