fix: add signature_of specialization for r-value references (#515)

* test: add Variant move test

* fix: provide signature_of<_T&&>

Provide signature_of specialization for rvalue references resp.
allow when forwarding references resolve to rvalue references

signature_of<_T&&> is needed e.g. for std::map::try_emplace() when the
emplaced sdbus:: type is passed as rvalue reference, otherwise the
static_assert "Unsupported D-Bus type ..." would trigger

See #513 (comment)
for an elaborate explanation

Signed-off-by: Simon Braunschmidt <simon.braunschmidt@iba-group.com>

---------

Signed-off-by: Simon Braunschmidt <simon.braunschmidt@iba-group.com>
Co-authored-by: Stanislav Angelovic <stanislav.angelovic.ext@siemens.com>

Author: SimonBraunschmidtIBA

include/sdbus-c++/TypeTraits.h

@@ -151,6 +151,10 @@ namespace sdbus {
     struct signature_of<_T&> : signature_of<_T>
     {};
 
+    template <typename _T>
+    struct signature_of<_T&&> : signature_of<_T>
+    {};
+
     template <>
     struct signature_of<void>
     {

tests/unittests/Types_test.cpp

@@ -97,6 +97,29 @@ TEST(AVariant, CanBeCopied)
     ASSERT_THAT(variantCopy2.get<std::string>(), Eq(value));
 }
 
+TEST(AVariant, CanBeMoved)
+{
+    auto value = "hello"s;
+    sdbus::Variant variant(value);
+
+    auto movedVariant{std::move(variant)};
+
+    ASSERT_THAT(movedVariant.get<std::string>(), Eq(value));
+    ASSERT_TRUE(variant.isEmpty());
+}
+
+TEST(AVariant, CanBeMovedIntoAMap)
+{
+    auto value = "hello"s;
+    sdbus::Variant variant(value);
+
+    std::map<std::string, sdbus::Variant> mymap;
+    mymap.try_emplace("payload", std::move(variant));
+
+    ASSERT_THAT(mymap["payload"].get<std::string>(), Eq(value));
+    ASSERT_TRUE(variant.isEmpty());
+}
+
 TEST(AVariant, IsNotEmptyWhenContainsAValue)
 {
     sdbus::Variant v("hello");