Vk: make variadic extension type traits work for an empty list as well.

src/Magnum/Vk/Test/ExtensionsTest.cpp

@@ -81,6 +81,9 @@ void ExtensionsTest::isInstanceExtension() {
         Extensions::KHR::get_physical_device_properties2,
         Extensions::KHR::external_memory_capabilities,
         Extensions::KHR::external_fence>::value)); /* not */
+
+    /* Empty variadic list should return true */
+    CORRADE_VERIFY(Implementation::IsInstanceExtension<>::value);
 }
 
 void ExtensionsTest::isExtension() {
@@ -117,6 +120,9 @@ void ExtensionsTest::isExtension() {
         Extensions::KHR::external_memory,
         Extensions::KHR::depth_stencil_resolve,
         Extensions::KHR::external_fence_capabilities>::value)); /* not */
+
+    /* Empty variadic list should return true */
+    CORRADE_VERIFY(Implementation::IsExtension<>::value);
 }
 
 void ExtensionsTest::constructInstanceExtensionFromCompileTimeExtension() {

src/Magnum/Vk/TypeTraits.h

@@ -40,6 +40,7 @@ namespace Implementation {
     /** @todo filter out GL/AL extensions also */
 
     template<class...> class IsInstanceExtension;
+    template<> class IsInstanceExtension<> { public: enum: bool { value = true }; };
     CORRADE_HAS_TYPE(IsInstanceExtension<U>, decltype(T::InstanceIndex));
     template<class T, class U, class ...Args> class IsInstanceExtension<T, U, Args...> {
         /** @todo C++17: use &&... instead of all this */
@@ -47,6 +48,7 @@ namespace Implementation {
     };
 
     template<class...> class IsExtension;
+    template<> class IsExtension<> { public: enum: bool { value = true }; };
     CORRADE_HAS_TYPE(IsExtension<U>, decltype(T::Index));
     template<class T, class U, class ...Args> class IsExtension<T, U, Args...> {
         /** @todo C++17: use &&... instead of all this */