make add command working with latest tag if offline or preferOffline …

…is setted (#2521)

__tests__/commands/add.js

@@ -630,9 +630,38 @@ test.skip('add asks for correct package version if user passes an incorrect one'
 
 test.concurrent('install with latest tag', (): Promise<void> => {
   return runAdd(['left-pad@latest'], {}, 'latest-version-in-package', async (config) => {
+    const lockfile = explodeLockfile(await fs.readFile(path.join(config.cwd, 'yarn.lock')));
+    const pkg = await fs.readJson(path.join(config.cwd, 'package.json'));
+    const version = await getPackageVersion(config, 'left-pad');
+
+    assert.deepEqual(pkg.dependencies, {'left-pad': `^${version}`});
+    assert(lockfile.indexOf(`left-pad@^${version}:`) === 0);
+  });
+});
+
+test.concurrent('install with latest tag and --offline flag', (): Promise<void> => {
+  return runAdd(['left-pad@latest'], {}, 'latest-version-in-package', async (config, reporter, previousAdd) => {
+    config.offline = true;
+    const add = new Add(['left-pad@latest'], {}, config, reporter, previousAdd.lockfile);
+    await add.init();
+
+    const pkg = await fs.readJson(path.join(config.cwd, 'package.json'));
+    const version = await getPackageVersion(config, 'left-pad');
+
+    assert.deepEqual(pkg.dependencies, {'left-pad': `^${version}`});
+  });
+});
+
+test.concurrent('install with latest tag and --prefer-offline flag', (): Promise<void> => {
+  return runAdd(['[email protected]'], {}, 'latest-version-in-package', async (config, reporter, previousAdd) => {
+    config.preferOffline = true;
+    const add = new Add(['left-pad@latest'], {}, config, reporter, previousAdd.lockfile);
+    await add.init();
+
     const pkg = await fs.readJson(path.join(config.cwd, 'package.json'));
     const version = await getPackageVersion(config, 'left-pad');
 
     assert.deepEqual(pkg.dependencies, {'left-pad': `^${version}`});
+    assert.notEqual(version, '1.1.0');
   });
 });

src/package-constraint-resolver.js

@@ -19,6 +19,11 @@ export default class PackageConstraintResolver {
   config: Config;
 
   reduce(versions: Array<string>, range: string): Promise<?string> {
-    return Promise.resolve(semver.maxSatisfying(versions, range, this.config.looseSemver));
+    if (range === 'latest') {
+      // Usually versions are already ordered and the last one is the latest
+      return Promise.resolve(versions[versions.length - 1]);
+    } else {
+      return Promise.resolve(semver.maxSatisfying(versions, range, this.config.looseSemver));
+    }
   }
 }