Allow mocking closures

Mocking closures is doable today by mocking __invoke on an invokable class. However this is troublesome, as it requires writing an invokable class, and mocking a class method.

This new helper makes things easier via a new `$this->createClosureMock()` method.

src/Framework/MockObject/Runtime/Stub/ClosureMock.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace PHPUnit\Framework\MockObject\Stub;
+
+use PHPUnit\Framework\MockObject\Builder\InvocationMocker;
+use PHPUnit\Framework\MockObject\Rule\InvocationOrder;
+
+class ClosureMock
+{
+    public function __invoke()
+    {
+    }
+
+    public function expectsClosure(InvocationOrder $invocationRule): InvocationMocker
+    {
+        return $this->expects($invocationRule)
+            ->method('__invoke');
+    }
+
+    public function closure(): InvocationMocker
+    {
+        return $this->method('__invoke');
+    }
+}

src/Framework/TestCase.php

@@ -78,6 +78,7 @@
 use PHPUnit\Framework\MockObject\Rule\InvokedAtMostCount as InvokedAtMostCountMatcher;
 use PHPUnit\Framework\MockObject\Rule\InvokedCount as InvokedCountMatcher;
 use PHPUnit\Framework\MockObject\Stub;
+use PHPUnit\Framework\MockObject\Stub\ClosureMock;
 use PHPUnit\Framework\MockObject\Stub\ConsecutiveCalls as ConsecutiveCallsStub;
 use PHPUnit\Framework\MockObject\Stub\Exception as ExceptionStub;
 use PHPUnit\Framework\MockObject\Stub\ReturnArgument as ReturnArgumentStub;
@@ -1387,6 +1388,17 @@ final protected function createPartialMock(string $originalClassName, array $met
         return $partialMock;
     }
 
+    /**
+     * Creates mock of a closure.
+     *
+     * @throws InvalidArgumentException
+     * @throws MockObjectException
+     */
+    final protected function createClosureMock(): MockObject|ClosureMock
+    {
+        return $this->createPartialMock(ClosureMock::class, ['__invoke']);
+    }
+
     /**
      * Creates a test proxy for the specified class.
      *

tests/unit/Framework/MockObject/Creation/CreateClosureMockTest.php

@@ -0,0 +1,88 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace PHPUnit\Framework\MockObject;
+
+use PHPUnit\Framework\Attributes\Group;
+use PHPUnit\Framework\Attributes\Medium;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\MockObject\Generator\ClassIsFinalException;
+use PHPUnit\Framework\MockObject\Stub\ClosureMock;
+use PHPUnit\Framework\TestCase;
+use PHPUnit\TestFixture\MockObject\ExtendableClass;
+use PHPUnit\TestFixture\MockObject\InterfaceWithReturnTypeDeclaration;
+use ReflectionProperty;
+
+#[Group('test-doubles')]
+#[Group('test-doubles/creation')]
+#[Group('test-doubles/mock-object')]
+#[Medium]
+#[TestDox('createClosureMock()')]
+final class CreateClosureMockTest extends TestCase
+{
+    public function testCreateClosureMock(): void
+    {
+        $mock = $this->createClosureMock();
+
+        $this->assertInstanceOf(ClosureMock::class, $mock);
+        $this->assertInstanceOf(Stub::class, $mock);
+    }
+
+    public function testCreateClosureMockWithReturnValue(): void
+    {
+        $mock = $this->createClosureMock();
+
+        $mock->closure()->willReturn(123);
+
+        $this->assertSame(123, $mock());
+    }
+
+    public function testCreateClosureMockWithExpectation(): void
+    {
+        $mock = $this->createClosureMock();
+
+        $mock->expectsClosure($this->once())
+                ->willReturn(123);
+
+        $this->assertSame(123, $mock());
+    }
+
+    public function testClosureMockAppliesExpects(): void
+    {
+        $mock = $this->createClosureMock();
+
+        $mock->expectsClosure($this->once());
+
+        $this->assertThatMockObjectExpectationFails(
+            "Expectation failed for method name is \"__invoke\" when invoked 1 time.\nMethod was expected to be called 1 time, actually called 0 times.\n",
+            $mock,
+        );
+    }
+
+    private function assertThatMockObjectExpectationFails(string $expectationFailureMessage, MockObject $mock, string $methodName = '__phpunit_verify', array $arguments = []): void
+    {
+        try {
+            call_user_func_array([$mock, $methodName], $arguments);
+        } catch (ExpectationFailedException|MatchBuilderNotFoundException $e) {
+            $this->assertSame($expectationFailureMessage, $e->getMessage());
+
+            return;
+        } finally {
+            $this->resetMockObjects();
+        }
+
+        $this->fail();
+    }
+
+    private function resetMockObjects(): void
+    {
+        (new ReflectionProperty(TestCase::class, 'mockObjects'))->setValue($this, []);
+    }
+}