Math: test constexpr-ness of subclass types.

Fails with libc++ because std::forward() is not constexpr in C++11 (yes,
I'm adding the test solely because of that).

src/Magnum/Math/Test/MatrixTest.cpp

@@ -326,8 +326,9 @@ void MatrixTest::subclassTypes() {
 }
 
 void MatrixTest::subclass() {
-    const Mat2 a(Vec2(2.0f, 3.5f),
+    /* Constexpr constructor */
-                 Vec2(3.0f, 1.0f));
+    constexpr Mat2 a(Vec2(2.0f, 3.5f),
+                     Vec2(3.0f, 1.0f));
     Mat2 b(Vec2(2.0f, 3.5f),
            Vec2(3.0f, 1.0f));
     CORRADE_COMPARE(a[1], Vec2(3.0f, 1.0f));

src/Magnum/Math/Test/RangeTest.cpp

@@ -466,6 +466,10 @@ void RangeTest::subclassTypes() {
 }
 
 void RangeTest::subclass() {
+    /* Constexpr constructor */
+    constexpr Recti a{Vector2i{34, 23}, Vector2i{47, 30}};
+    CORRADE_COMPARE(a.min(), (Vector2i{34, 23}));
+
     CORRADE_COMPARE(Recti::fromSize({3, 5}, {23, 78}),
                     Recti(Vector2i{3, 5}, Vector2i{26, 83}));
 

src/Magnum/Math/Test/RectangularMatrixTest.cpp

@@ -500,8 +500,9 @@ void RectangularMatrixTest::subclass() {
     CORRADE_COMPARE(Mat2x2::fromDiagonal({1.0f, -2.0f}), Mat2x2(Vector2(1.0f,  0.0f),
                                                                 Vector2(0.0f, -2.0f)));
 
-    const Mat2x2 a(Vector2(1.0f, -3.0f),
+    /* Constexpr constructor */
-                   Vector2(-3.0f, 1.0f));
+    constexpr Mat2x2 a(Vector2(1.0f, -3.0f),
+                       Vector2(-3.0f, 1.0f));
     CORRADE_COMPARE(-a, Mat2x2(Vector2(-1.0f, 3.0f),
                                Vector2(3.0f, -1.0f)));
 

src/Magnum/Math/Test/VectorTest.cpp

@@ -562,6 +562,10 @@ void VectorTest::subclass() {
         CORRADE_COMPARE(c, Vec2(5.0f, -1.0f));
     }
 
+    /* Constexpr constructor */
+    constexpr const Vec2 a{-2.0f, 5.0f};
+    CORRADE_COMPARE(a[0], -2.0f);
+
     CORRADE_COMPARE(Vec2(-2.0f, 5.0f) + Vec2(1.0f, -3.0f), Vec2(-1.0f, 2.0f));
     CORRADE_COMPARE(Vec2(-2.0f, 5.0f) - Vec2(1.0f, -3.0f), Vec2(-3.0f, 8.0f));