MSVC 2015 compatibility: random stuff that's causing compiler crashes.

Random indeed. In particular, it's causing crashes for Vector2 and
Vector3, but not for the exact same code in Vector4!

src/Magnum/Math/Range.h

@@ -259,7 +259,14 @@ template<class T> class Range2D: public Range<2, T> {
          * @brief Construct range from external representation
          * @todoc Remove workaround when Doxygen no longer chokes on that line
          */
-        template<class U, class V = decltype(Implementation::RangeConverter<2, T, U>::from(std::declval<U>()))> constexpr explicit Range2D(const U& other)
+        template<class U, class V =
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Causes ICE */
+            decltype(Implementation::RangeConverter<2, T, U>::from(std::declval<U>()))
+            #else
+            decltype(Implementation::RangeConverter<2, T, U>())
+            #endif
+            >
+        constexpr explicit Range2D(const U& other)
             #ifndef DOXYGEN_GENERATING_OUTPUT
             /* MSVC 2015 can't handle {} here */
             : Range<2, T>(Implementation::RangeConverter<2, T, U>::from(other))

src/Magnum/Math/Vector2.h

@@ -159,7 +159,14 @@ template<class T> class Vector2: public Vector<2, T> {
             {}
 
         /** @brief Construct vector from external representation */
-        template<class U, class V = decltype(Implementation::VectorConverter<2, T, U>::from(std::declval<U>()))> constexpr explicit Vector2(const U& other): Vector<2, T>(Implementation::VectorConverter<2, T, U>::from(other)) {}
+        template<class U, class V =
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Causes ICE */
+            decltype(Implementation::VectorConverter<2, T, U>::from(std::declval<U>()))
+            #else
+            decltype(Implementation::VectorConverter<2, T, U>())
+            #endif
+            >
+        constexpr explicit Vector2(const U& other): Vector<2, T>(Implementation::VectorConverter<2, T, U>::from(other)) {}
 
         /** @brief Copy constructor */
         constexpr Vector2(const Vector<2, T>& other): Vector<2, T>(other) {}

src/Magnum/Math/Vector3.h

@@ -190,7 +190,14 @@ template<class T> class Vector3: public Vector<3, T> {
             {}
 
         /** @brief Construct vector from external representation */
-        template<class U, class V = decltype(Implementation::VectorConverter<3, T, U>::from(std::declval<U>()))> constexpr explicit Vector3(const U& other): Vector<3, T>(Implementation::VectorConverter<3, T, U>::from(other)) {}
+        template<class U, class V =
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Causes ICE */
+            decltype(Implementation::VectorConverter<3, T, U>::from(std::declval<U>()))
+            #else
+            decltype(Implementation::VectorConverter<3, T, U>())
+            #endif
+            >
+        constexpr explicit Vector3(const U& other): Vector<3, T>(Implementation::VectorConverter<3, T, U>::from(other)) {}
 
         /** @brief Copy constructor */
         constexpr Vector3(const Vector<3, T>& other): Vector<3, T>(other) {}