[9.x] Allow terser singleton bindings

src/Illuminate/Foundation/Application.php

@@ -696,6 +696,8 @@ public function register($provider, $force = false)
 
         if (property_exists($provider, 'singletons')) {
             foreach ($provider->singletons as $key => $value) {
+                $key = is_int($key) ? $value : $key;
+
                 $this->singleton($key, $value);
             }
         }

tests/Foundation/FoundationApplicationTest.php

@@ -67,6 +67,7 @@ public function testSingletonsAreCreatedWhenServiceProviderIsRegistered()
         $app->register($provider = new class($app) extends ServiceProvider
         {
             public $singletons = [
+                NonContractBackedClass::class,
                 AbstractClass::class => ConcreteClass::class,
             ];
         });
@@ -77,6 +78,11 @@ public function testSingletonsAreCreatedWhenServiceProviderIsRegistered()
 
         $this->assertInstanceOf(ConcreteClass::class, $instance);
         $this->assertSame($instance, $app->make(AbstractClass::class));
+
+        $instance = $app->make(NonContractBackedClass::class);
+
+        $this->assertInstanceOf(NonContractBackedClass::class, $instance);
+        $this->assertSame($instance, $app->make(NonContractBackedClass::class));
     }
 
     public function testServiceProvidersAreCorrectlyRegisteredWhenRegisterMethodIsNotFilled()
@@ -613,6 +619,11 @@ class ConcreteClass extends AbstractClass
     //
 }
 
+class NonContractBackedClass
+{
+    //
+}
+
 class ConcreteTerminator
 {
     public static $counter = 0;