fix(vitest): don't hang with maxConcurrency 0

packages/vitest/src/node/config/resolveConfig.ts

@@ -207,6 +207,13 @@ export function resolveConfig(
     resolved.minWorkers = 1
   }
 
+  if (resolved.maxConcurrency === 0) {
+    logger.console.warn(
+      c.yellow(`The option "maxConcurrency" cannot be set to 0. Using default value ${configDefaults.maxConcurrency} instead.`),
+    )
+    resolved.maxConcurrency = configDefaults.maxConcurrency
+  }
+
   if (resolved.inspect || resolved.inspectBrk) {
     const isSingleThread
       = resolved.pool === 'threads'

test/config/test/failures.test.ts

@@ -279,3 +279,10 @@ test('mergeReports doesn\'t work with watch mode enabled', async () => {
 
   expect(stderr).toMatch('Cannot merge reports with --watch enabled')
 })
+
+test('maxConcurrency 0 prints a warning', async () => {
+  const { stderr, ctx } = await runVitest({ maxConcurrency: 0 })
+
+  expect(ctx?.config.maxConcurrency).toBe(5)
+  expect(stderr).toMatch('The option "maxConcurrency" cannot be set to 0. Using default value 5 instead.')
+})

test/config/vitest.config.ts

@@ -19,6 +19,7 @@ export default defineConfig({
       truncateThreshold: 999,
     },
     coverage: {
+      // test that empty reporter does not throw
       reporter: [],
     },
   },