Deprecate implicit conversion of Resource<T, U> to U&.

We are allowing implicit conversions only if they are harmless and this
is *not* harmless, as it might fire an assertion. The user should use
operator*() or operator->() instead, which will make these conversions
stand out more in the code. Also reduced code duplication.

src/Magnum/DebugTools/Implementation/CapsuleRenderer.cpp

@@ -45,21 +45,21 @@ AbstractCapsuleRenderer<2>::AbstractCapsuleRenderer(): AbstractShapeRenderer<2>(
 
     /* Bottom hemisphere */
     if(!(bottom = ResourceManager::instance().get<MeshView>("capsule2d-bottom"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(0, rings*4, 0, rings*2+1);
         ResourceManager::instance().set(bottom.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }
 
     /* Cylinder */
     if(!(cylinder = ResourceManager::instance().get<MeshView>("capsule2d-cylinder"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(rings*4, 4, rings*2+1, rings*2+3);
         ResourceManager::instance().set(cylinder.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }
 
     /* Top hemisphere */
     if(!(top = ResourceManager::instance().get<MeshView>("capsule2d-top"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(rings*4+4, rings*4, rings*2+3, rings*4+4);
         ResourceManager::instance().set(top.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }
@@ -72,21 +72,21 @@ AbstractCapsuleRenderer<3>::AbstractCapsuleRenderer(): AbstractShapeRenderer<3>(
 
     /* Bottom hemisphere */
     if(!(bottom = ResourceManager::instance().get<MeshView>("capsule3d-bottom"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(0, rings*8, 0, rings*4+1);
         ResourceManager::instance().set(bottom.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }
 
     /* Cylinder */
     if(!(cylinder = ResourceManager::instance().get<MeshView>("capsule3d-cylinder"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(rings*8, segments*4+8, rings*4+1, rings*4+segments*2+5);
         ResourceManager::instance().set(cylinder.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }
 
     /* Top */
     if(!(top = ResourceManager::instance().get<MeshView>("capsule3d-top"))) {
-        auto view = new MeshView(wireframeMesh);
+        auto view = new MeshView(*wireframeMesh);
         view->setIndexRange(rings*8+segments*4+8, rings*8, rings*4+segments*2+5, rings*8+segments*2+6);
         ResourceManager::instance().set(top.key(), view, ResourceDataState::Final, ResourcePolicy::Manual);
     }

src/Magnum/Resource.h

@@ -191,28 +191,39 @@ class Resource {
          * @brief Reference to resource data
          *
          * The resource must be loaded before accessing it. Use boolean
-         * conversion operator or state() for testing whether it is loaded.
+         * conversion operator or @ref state() for testing whether it is
+         * loaded.
          */
-        operator U&() {
+        U& operator*() {
             acquire();
             CORRADE_ASSERT(data, "Resource: accessing not loaded data with key" << key(), *static_cast<U*>(data));
             return *static_cast<U*>(data);
         }
 
-        /** @overload */
+        #ifdef MAGNUM_BUILD_DEPRECATED
+        /**
+         * @overload
+         * @deprecated Use the explicit @ref Magnum::Resource::operator*() "operator*()" or
+         *      @ref Magnum::Resource::operator->() "operator->()" instead.
+         *      Implicit conversion is no longer allowed if it might throw an
+         *      assertion.
+         */
+        CORRADE_DEPRECATED("use operator*() or operator->() instead") operator U&() { return **this; }
+        #endif
+
+        /**
+         * @brief Access to resource data
+         *
+         * The resource must be loaded before accessing it. Use boolean
+         * conversion operator or @ref state() for testing whether it is
+         * loaded.
+         */
         U* operator->() {
             acquire();
             CORRADE_ASSERT(data, "Resource: accessing not loaded data with key" << key(), nullptr);
             return static_cast<U*>(data);
         }
 
-        /** @overload */
-        U& operator*() {
-            acquire();
-            CORRADE_ASSERT(data, "Resource: accessing not loaded data with key" << key(), *static_cast<U*>(data));
-            return *static_cast<U*>(data);
-        }
-
     private:
         Resource(Implementation::ResourceManagerData<T>* manager, ResourceKey key): manager(manager), _key(key), lastCheck(0), _state(ResourceState::NotLoaded), data(nullptr) {
             manager->incrementReferenceCount(key);