Warn on package lock json

__tests__/commands/install/lockfiles.js

@@ -326,3 +326,19 @@ test.concurrent("install should fix if lockfile patterns don't match resolved ve
     expect(lockContent).toContain('left-pad-1.1.2.tgz');
   });
 });
+
+test.concurrent('install should warn if a conflicting npm package-lock.json exists', (): Promise<void> => {
+  const fixture = 'lockfile-conflict-package-lock-json';
+
+  return runInstall({}, fixture, (config, reporter, install, getStdout) => {
+    expect(getStdout()).toContain('package-lock.json found');
+  });
+});
+
+test.concurrent('install should warn if a conflicting npm npm-shrinkwrap.json exists', (): Promise<void> => {
+  const fixture = 'lockfile-conflict-npm-shrinkwrap-json';
+
+  return runInstall({}, fixture, (config, reporter, install, getStdout) => {
+    expect(getStdout()).toContain('npm-shrinkwrap.json found');
+  });
+});

__tests__/fixtures/install/lockfile-conflict-npm-shrinkwrap-json/package.json

@@ -0,0 +1,5 @@
+{
+  "dependencies": {
+    "left-pad": "^1.1.3"
+  }
+}

__tests__/fixtures/install/lockfile-conflict-package-lock-json/package.json

@@ -0,0 +1,5 @@
+{
+  "dependencies": {
+    "left-pad": "^1.1.3"
+  }
+}

src/cli/commands/install.js

@@ -516,10 +516,15 @@ export class Install {
     this.checkUpdate();
 
     // warn if we have a shrinkwrap
-    if (await fs.exists(path.join(this.config.lockfileFolder, 'npm-shrinkwrap.json'))) {
+    if (await fs.exists(path.join(this.config.lockfileFolder, constants.NPM_SHRINKWRAP_FILENAME))) {
       this.reporter.warn(this.reporter.lang('shrinkwrapWarning'));
     }
 
+    // warn if we have an npm lockfile
+    if (await fs.exists(path.join(this.config.lockfileFolder, constants.NPM_LOCK_FILENAME))) {
+      this.reporter.warn(this.reporter.lang('npmLockfileWarning'));
+    }
+
     // running a focused install in a workspace root is not allowed
     if (this.flags.focus && (!this.config.workspaceRootFolder || this.config.cwd === this.config.workspaceRootFolder)) {
       throw new MessageError(this.reporter.lang('workspacesFocusRootCheck'));

src/constants.js

@@ -86,6 +86,7 @@ export const TARBALL_FILENAME = '.yarn-tarball.tgz';
 export const CLEAN_FILENAME = '.yarnclean';
 
 export const NPM_LOCK_FILENAME = 'package-lock.json';
+export const NPM_SHRINKWRAP_FILENAME = 'npm-shrinkwrap.json';
 
 export const DEFAULT_INDENT = '  ';
 export const SINGLE_INSTANCE_PORT = 31997;

src/reporters/lang/en.js

@@ -94,6 +94,8 @@ const messages = {
   couldntFindManifestIn: "Couldn't find manifest in $0.",
   shrinkwrapWarning:
     'npm-shrinkwrap.json found. This will not be updated or respected. See https://yarnpkg.com/en/docs/migrating-from-npm for more information.',
+  npmLockfileWarning:
+    'package-lock.json found. Your project contains lock files generated by tools other than Yarn. It is advised not to mix package managers in order to avoid resolution inconsistencies caused by unsynchronized lock files. To clear this warning, remove package-lock.json.',
   lockfileOutdated: 'Outdated lockfile. Please run `yarn install` and try again.',
   lockfileMerged: 'Merge conflict detected in yarn.lock and successfully merged.',
   lockfileConflict: