Math: test that implicit conversion from/to foreign types can't compile.

src/Math/Test/Vector2Test.cpp

@@ -144,6 +144,10 @@ void Vector2Test::convert() {
     Vec2 d(b);
     CORRADE_COMPARE(d.x, a.x);
     CORRADE_COMPARE(d.y, a.y);
+
+    /* Implicit conversion is not allowed */
+    CORRADE_VERIFY(!(std::is_convertible<Vec2, Vector2>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Vector2, Vec2>::value));
 }
 
 void Vector2Test::access() {

src/Math/Test/Vector3Test.cpp

@@ -158,6 +158,10 @@ void Vector3Test::convert() {
     CORRADE_COMPARE(d.x, a.x);
     CORRADE_COMPARE(d.y, a.y);
     CORRADE_COMPARE(d.z, a.z);
+
+    /* Implicit conversion is not allowed */
+    CORRADE_VERIFY(!(std::is_convertible<Vec3, Vector3>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Vector3, Vec3>::value));
 }
 
 void Vector3Test::access() {

src/Math/Test/Vector4Test.cpp

@@ -156,6 +156,10 @@ void Vector4Test::convert() {
     CORRADE_COMPARE(d.y, a.y);
     CORRADE_COMPARE(d.z, a.z);
     CORRADE_COMPARE(d.w, a.w);
+
+    /* Implicit conversion is not allowed */
+    CORRADE_VERIFY(!(std::is_convertible<Vec4, Vector4>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Vector4, Vec4>::value));
 }
 
 void Vector4Test::access() {

src/Math/Test/VectorTest.cpp

@@ -220,6 +220,10 @@ void VectorTest::convert() {
     CORRADE_COMPARE(d.x, a.x);
     CORRADE_COMPARE(d.y, a.y);
     CORRADE_COMPARE(d.z, a.z);
+
+    /* Implicit conversion is not allowed */
+    CORRADE_VERIFY(!(std::is_convertible<Vec3, Vector3>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Vector3, Vec3>::value));
 }
 
 void VectorTest::data() {