fix(use): create package.json when calling corepack use on empty dir

sources/commands/Base.ts

@@ -37,7 +37,7 @@ export abstract class BaseCommand extends Command<Context> {
   async setLocalPackageManager(info: PreparedPackageManagerInfo) {
     const lookup = await specUtils.loadSpec(this.context.cwd);
 
-    const content = lookup.target !== `NoProject`
+    const content = lookup.type !== `NoProject`
       ? await fs.promises.readFile(lookup.target, `utf8`)
       : ``;
 

sources/commands/Use.ts

@@ -16,7 +16,7 @@ export class UseCommand extends BaseCommand {
     `,
     examples: [[
       `Configure the project to use the latest Yarn release`,
-      `corepack use 'yarn@*'`,
+      `corepack use 'yarn'`,
     ]],
   });
 

sources/specUtils.ts

@@ -3,6 +3,7 @@ import fs                                               from 'fs';
 import path                                             from 'path';
 import semver                                           from 'semver';
 
+import {NodeError}                                      from './nodeUtils';
 import {Descriptor, Locator, isSupportedPackageManager} from './types';
 
 const nodeModulesRegExp = /[\\/]node_modules[\\/](@[^\\/]*[\\/])?([^@\\/][^\\/]*)$/;
@@ -88,6 +89,8 @@ export async function loadSpec(initialCwd: string): Promise<LoadSpecResult> {
     manifestPath: string;
   } | null = null;
 
+  let content: string;
+
   while (nextCwd !== currCwd && (!selection || !selection.data.packageManager)) {
     currCwd = nextCwd;
     nextCwd = path.dirname(currCwd);
@@ -96,10 +99,12 @@ export async function loadSpec(initialCwd: string): Promise<LoadSpecResult> {
       continue;
 
     const manifestPath = path.join(currCwd, `package.json`);
-    if (!fs.existsSync(manifestPath))
-      continue;
-
-    const content = await fs.promises.readFile(manifestPath, `utf8`);
+    try {
+      content = await fs.promises.readFile(manifestPath, `utf8`);
+    } catch (err) {
+      if ((err as NodeError)?.code === `ENOENT`) continue;
+      throw err;
+    }
 
     let data;
     try {

tests/Use.test.ts

@@ -13,6 +13,7 @@ describe(`UseCommand`, () => {
   it(`should set the package manager in the current project`, async () => {
     await xfs.mktempPromise(async cwd => {
       await xfs.writeJsonPromise(ppath.join(cwd, `package.json`), {
+        packageManager: `yarn@1.0.0`,
       });
 
       await expect(runCli(cwd, [`use`, `yarn@1.22.4`])).resolves.toMatchObject({
@@ -29,4 +30,37 @@ describe(`UseCommand`, () => {
       });
     });
   });
+
+  it(`should create a package.json if absent`, async () => {
+    await xfs.mktempPromise(async cwd => {
+      await expect(runCli(cwd, [`use`, `yarn@1.22.4`])).resolves.toMatchObject({
+        exitCode: 0,
+        stderr: `warning package.json: No license field\nwarning No license field\n`,
+      });
+
+      await expect(xfs.readJsonPromise(ppath.join(cwd, `package.json`))).resolves.toMatchObject({
+        packageManager: `yarn@1.22.4+sha256.bc5316aa110b2f564a71a3d6e235be55b98714660870c5b6b2d2d3f12587fb58`,
+      });
+
+      await expect(runCli(cwd, [`yarn`, `--version`])).resolves.toMatchObject({
+        exitCode: 0,
+        stdout: `1.22.4\n`,
+        stderr: ``,
+      });
+
+      // Ensure Corepack is able to detect package.json in parent directory
+      const subfolder = ppath.join(cwd, `subfolder`);
+      await xfs.mkdirPromise(subfolder);
+
+      await expect(runCli(subfolder, [`use`, `yarn@2.2.2`])).resolves.toMatchObject({
+        exitCode: 0,
+        stderr: ``,
+      });
+      await expect(runCli(cwd, [`yarn`, `--version`])).resolves.toMatchObject({
+        exitCode: 0,
+        stdout: `2.2.2\n`,
+        stderr: ``,
+      });
+    });
+  });
 });