Math: make unary Vector operator+() constexpr.

No reason not to. Well, except MSVC 2015, because of course that one is
a special snowflake, FFS.

src/Magnum/Math/Test/VectorTest.cpp

@@ -400,10 +400,14 @@ void VectorTest::compareComponentWise() {
 }
 
 void VectorTest::promotedNegated() {
-    CORRADE_COMPARE(+Vector4(1.0f, -3.0f, 5.0f, -10.0f),
+    CORRADE_COMPARE(+(Vector4{1.0f, -3.0f, 5.0f, -10.0f}),
-                     Vector4(1.0f, -3.0f, 5.0f, -10.0f));
+                     (Vector4{1.0f, -3.0f, 5.0f, -10.0f}));
-    CORRADE_COMPARE(-Vector4(1.0f, -3.0f, 5.0f, -10.0f),
+    CORRADE_COMPARE(-(Vector4{1.0f, -3.0f, 5.0f, -10.0f}),
-                     Vector4(-1.0f, 3.0f, -5.0f, 10.0f));
+                     (Vector4{-1.0f, 3.0f, -5.0f, 10.0f}));
+
+    constexpr Vector4 a{1.0f, -3.0f, 5.0f, -10.0f};
+    constexpr Vector4 promotedA = +a;
+    CORRADE_COMPARE(promotedA, a);
 }
 
 void VectorTest::addSubtract() {
@@ -841,6 +845,15 @@ void VectorTest::subclass() {
 
     /* Unary operators */
     CORRADE_COMPARE(+Vec2(-2.0f, 5.0f), Vec2(-2.0f, 5.0f));
+    {
+        constexpr Vec2 ca{-2.0f, 5.0f};
+        #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Probably because copy is not constexpr */
+        constexpr
+        #endif
+        Vec2 cb = +ca;
+        CORRADE_COMPARE(cb, ca);
+    }
+
     CORRADE_COMPARE(-Vec2(-2.0f, 5.0f), Vec2(2.0f, -5.0f));
 
     /* Addition / subtraction */

src/Magnum/Math/Vector.h

@@ -345,7 +345,7 @@ template<std::size_t size, class T> class Vector {
          *
          * Returns the value as-is.
          */
-        Vector<size, T> operator+() const { return *this; }
+        constexpr Vector<size, T> operator+() const { return *this; }
 
         /**
          * @brief Negated vector
@@ -1281,7 +1281,7 @@ extern template MAGNUM_EXPORT Debug& operator<<(Debug&, const Vector<4, Double>&
         return Math::Vector<size, T>::pad(a, value);                        \
     }                                                                       \
                                                                             \
-    Type<T> operator+() const {                                             \
+    constexpr Type<T> operator+() const {                                   \
         return Math::Vector<size, T>::operator+();                          \
     }                                                                       \
     template<class U = T> typename std::enable_if<std::is_signed<U>::value, Type<T>>::type \