MSVC 2015 compatibility: single value Math constructor constexpr issues.

Some could be worked around, but the rest is prevented by inability to
delegate constexpr constructors.

src/Magnum/Math/BoolVector.h

@@ -93,7 +93,12 @@ template<std::size_t size> class BoolVector {
         #ifdef DOXYGEN_GENERATING_OUTPUT
         inline explicit BoolVector(T value);
         #else
-        template<class T, class U = typename std::enable_if<std::is_same<bool, T>::value && size != 1, bool>::type> constexpr explicit BoolVector(T value): BoolVector(typename Implementation::GenerateSequence<DataSize>::Type(), value ? FullSegmentMask : 0) {}
+        template<class T, class U = typename std::enable_if<std::is_same<bool, T>::value && size != 1, bool>::type>
+        #ifndef CORRADE_MSVC2015_COMPATIBILITY
+        /* Can't use delegating constructors with constexpr -- https://connect.microsoft.com/VisualStudio/feedback/details/1579279/c-constexpr-does-not-work-with-delegating-constructors */
+        constexpr
+        #endif
+        explicit BoolVector(T value): BoolVector(typename Implementation::GenerateSequence<DataSize>::Type(), value ? FullSegmentMask : 0) {}
         #endif
 
         /** @brief Copy constructor */

src/Magnum/Math/Test/BoolVectorTest.cpp

@@ -99,10 +99,18 @@ void BoolVectorTest::constructNoInit() {
 }
 
 void BoolVectorTest::constructOneValue() {
-    constexpr BoolVector19 a(false);
+    #ifndef CORRADE_MSVC2015_COMPATIBILITY
+    /* Can't use delegating constructors with constexpr -- https://connect.microsoft.com/VisualStudio/feedback/details/1579279/c-constexpr-does-not-work-with-delegating-constructors */
+    constexpr
+    #endif
+    BoolVector19 a(false);
     CORRADE_COMPARE(a, BoolVector19(0x00, 0x00, 0x00));
 
-    constexpr BoolVector19 b(true);
+    #ifndef CORRADE_MSVC2015_COMPATIBILITY
+    /* Can't use delegating constructors with constexpr -- https://connect.microsoft.com/VisualStudio/feedback/details/1579279/c-constexpr-does-not-work-with-delegating-constructors */
+    constexpr
+    #endif
+    BoolVector19 b(true);
     CORRADE_COMPARE(b, BoolVector19(0xff, 0xff, 0x07));
 
     CORRADE_VERIFY(!(std::is_convertible<bool, BoolVector19>::value));

src/Magnum/Math/Test/VectorTest.cpp

@@ -201,7 +201,11 @@ void VectorTest::constructNoInit() {
 }
 
 void VectorTest::constructOneValue() {
-    constexpr Vector4 a(7.25f);
+    #ifndef CORRADE_MSVC2015_COMPATIBILITY
+    /* Can't use delegating constructors with constexpr -- https://connect.microsoft.com/VisualStudio/feedback/details/1579279/c-constexpr-does-not-work-with-delegating-constructors */
+    constexpr
+    #endif
+    Vector4 a(7.25f);
 
     CORRADE_COMPARE(a, Vector4(7.25f, 7.25f, 7.25f, 7.25f));
 

src/Magnum/Math/Vector2.h

@@ -130,7 +130,14 @@ template<class T> class Vector2: public Vector<2, T> {
             {}
 
         /** @copydoc Vector::Vector(T) */
-        constexpr explicit Vector2(T value): Vector<2, T>(value) {}
+        constexpr explicit Vector2(T value):
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY
+            Vector<2, T>(value)
+            #else
+            /* Avoid using non-constexpr version */
+            Vector<2, T>(value, value)
+            #endif
+            {}
 
         /**
          * @brief Constructor

src/Magnum/Math/Vector3.h

@@ -152,7 +152,14 @@ template<class T> class Vector3: public Vector<3, T> {
             {}
 
         /** @copydoc Vector::Vector(T) */
-        constexpr explicit Vector3(T value): Vector<3, T>(value) {}
+        constexpr explicit Vector3(T value):
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY
+            Vector<3, T>(value)
+            #else
+            /* Avoid using non-constexpr version */
+            Vector<3, T>(value, value, value)
+            #endif
+            {}
 
         /**
          * @brief Constructor

src/Magnum/Math/Vector4.h

@@ -78,7 +78,14 @@ template<class T> class Vector4: public Vector<4, T> {
             {}
 
         /** @copydoc Vector::Vector(T) */
-        constexpr explicit Vector4(T value): Vector<4, T>(value) {}
+        constexpr explicit Vector4(T value):
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY
+            Vector<4, T>(value)
+            #else
+            /* Avoid using non-constexpr version */
+            Vector<4, T>(value, value, value, value)
+            #endif
+            {}
 
         /**
          * @brief Constructor