Trade: make AbstractImporter::defaultScene() const.

Makes more sense as the function isn't expected to fail (and thus any
kind of lazy population is not possible as it would be too late for
error checks anyway).

*Not* updating interface strings even though this is an ABI break
because we're doing that right after the skin import interface bump.

doc/changelog.dox

@@ -248,6 +248,12 @@ See also:
     @cpp Trade::AbstractMaterialData @ce aliases to, doesn't have a
     @cpp virtual @ce destructor as subclasses with extra data members aren't a
     desired use case anymore.
+-   @ref Trade::AbstractImporter::doDefaultScene() is now @cpp const @ce ---
+    since the function is not expected to fail, no kind of complex lazy
+    population can be done anyway. This is now consistent with `do*Count()`
+    interfaces, which are also @cpp const @ce and can't fail. Documentation of
+    each function was expanded to suggest a recommended place for potential
+    error handling.
 
 @section changelog-2020-06 2020.06
 

src/Magnum/Trade/AbstractImporter.cpp

@@ -212,12 +212,12 @@ void AbstractImporter::close() {
     }
 }
 
-Int AbstractImporter::defaultScene() {
+Int AbstractImporter::defaultScene() const {
     CORRADE_ASSERT(isOpened(), "Trade::AbstractImporter::defaultScene(): no file opened", -1);
     return doDefaultScene();
 }
 
-Int AbstractImporter::doDefaultScene() { return -1; }
+Int AbstractImporter::doDefaultScene() const { return -1; }
 
 UnsignedInt AbstractImporter::sceneCount() const {
     CORRADE_ASSERT(isOpened(), "Trade::AbstractImporter::sceneCount(): no file opened", 0);

src/Magnum/Trade/AbstractImporter.h

@@ -537,11 +537,8 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
          * When there is more than one scene, returns ID of the default one.
          * If there is no default scene, returns @cpp -1 @ce. Expects that a
          * file is opened.
-         *
-         * @note The function is not const, because the value will probably
-         *      be lazy-populated.
          */
-        Int defaultScene();
+        Int defaultScene() const;
 
         /**
          * @brief Scene count
@@ -1475,9 +1472,12 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
         /**
          * @brief Implementation for @ref defaultScene()
          *
-         * Default implementation returns @cpp -1 @ce.
+         * Default implementation returns @cpp -1 @ce. This function isn't
+         * expected to fail --- if an import error occus (for example because
+         * the default scene index is out of bounds), it should be handled
+         * already during file opening.
          */
-        virtual Int doDefaultScene();
+        virtual Int doDefaultScene() const;
 
         /**
          * @brief Implementation for @ref sceneCount()

src/Magnum/Trade/Test/AbstractImporterTest.cpp

@@ -1431,7 +1431,7 @@ void AbstractImporterTest::defaultScene() {
         bool doIsOpened() const override { return true; }
         void doClose() override {}
 
-        Int doDefaultScene() override { return 42; }
+        Int doDefaultScene() const override { return 42; }
     } importer;
 
     CORRADE_COMPARE(importer.defaultScene(), 42);

src/MagnumPlugins/AnySceneImporter/AnySceneImporter.cpp

@@ -165,7 +165,7 @@ Int AnySceneImporter::doAnimationForName(const std::string& name) { return _in->
 std::string AnySceneImporter::doAnimationName(const UnsignedInt id) { return _in->animationName(id); }
 Containers::Optional<AnimationData> AnySceneImporter::doAnimation(const UnsignedInt id) { return _in->animation(id); }
 
-Int AnySceneImporter::doDefaultScene() { return _in->defaultScene(); }
+Int AnySceneImporter::doDefaultScene() const { return _in->defaultScene(); }
 
 UnsignedInt AnySceneImporter::doSceneCount() const { return _in->sceneCount(); }
 Int AnySceneImporter::doSceneForName(const std::string& name) { return _in->sceneForName(name); }

src/MagnumPlugins/AnySceneImporter/AnySceneImporter.h

@@ -149,7 +149,7 @@ class MAGNUM_ANYSCENEIMPORTER_EXPORT AnySceneImporter: public AbstractImporter {
         MAGNUM_ANYSCENEIMPORTER_LOCAL Int doAnimationForName(const std::string& name) override;
         MAGNUM_ANYSCENEIMPORTER_LOCAL Containers::Optional<AnimationData> doAnimation(UnsignedInt id) override;
 
-        MAGNUM_ANYSCENEIMPORTER_LOCAL Int doDefaultScene() override;
+        MAGNUM_ANYSCENEIMPORTER_LOCAL Int doDefaultScene() const override;
 
         MAGNUM_ANYSCENEIMPORTER_LOCAL UnsignedInt doSceneCount() const override;
         MAGNUM_ANYSCENEIMPORTER_LOCAL Int doSceneForName(const std::string& name) override;