Math: be reasonable, <utility> is not needed for std::declval.

Despite what the standard tries to say. I bet a large portion of
<type_traits> is impossible to implement without it, which is why all
STL implementations define it there already.

src/Magnum/Math/Algorithms/KahanSum.h

@@ -29,7 +29,9 @@
  * @brief Function @ref Magnum::Math::Algorithms::kahanSum()
  */
 
-#include <utility> /* std::declval :( */
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 
 namespace Magnum { namespace Math { namespace Algorithms {
 

src/Magnum/Math/Bezier.h

@@ -30,6 +30,10 @@
  * @brief Class @ref Magnum::Math::Bezier, alias @ref Magnum::Math::QuadraticBezier, @ref Magnum::Math::QuadraticBezier2D, @ref Magnum::Math::QuadraticBezier3D, @ref Magnum::Math::CubicBezier, @ref Magnum::Math::CubicBezier2D, @ref Magnum::Math::CubicBezier3D
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Vector.h"
 
 #ifdef MAGNUM_BUILD_DEPRECATED

src/Magnum/Math/BitVector.h

@@ -30,6 +30,8 @@
  * @m_since_latest
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
 #include <type_traits>
 #include <Corrade/Containers/sequenceHelpers.h>
 #ifndef CORRADE_SINGLES_NO_DEBUG

src/Magnum/Math/Color.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Color3, @ref Magnum::Math::Color4, literal @link Magnum::Math::Literals::operator""_rgb() @endlink, @link Magnum::Math::Literals::operator""_rgba() @endlink, @link Magnum::Math::Literals::operator""_rgbf() @endlink, @link Magnum::Math::Literals::operator""_rgbaf() @endlink, @link Magnum::Math::Literals::operator""_srgb() @endlink, @link Magnum::Math::Literals::operator""_srgba() @endlink, @link Magnum::Math::Literals::operator""_srgbf() @endlink, @link Magnum::Math::Literals::operator""_srgbaf() @endlink
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Matrix.h"
 #include "Magnum/Math/Packing.h"
 #include "Magnum/Math/Vector4.h"

src/Magnum/Math/Complex.h

@@ -29,6 +29,9 @@
  * @brief Class @ref Magnum::Math::Complex, function @ref Magnum::Math::dot(), @ref Magnum::Math::angle()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #ifndef CORRADE_SINGLES_NO_DEBUG
 #include <Corrade/Utility/Debug.h>
 #endif

src/Magnum/Math/Dual.h

@@ -29,6 +29,9 @@
  * @brief Class @ref Magnum::Math::Dual
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #include <Corrade/Containers/Pair.h>
 #ifndef CORRADE_SINGLES_NO_DEBUG
 #include <Corrade/Utility/Debug.h>

src/Magnum/Math/DualComplex.h

@@ -30,6 +30,10 @@
  * @brief Class @ref Magnum::Math::DualComplex
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Complex.h"
 #include "Magnum/Math/Dual.h"
 #include "Magnum/Math/Matrix3.h"

src/Magnum/Math/DualQuaternion.h

@@ -30,6 +30,9 @@
  * @brief Class @ref Magnum::Math::DualQuaternion, function @ref Magnum::Math::sclerp()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #include <Corrade/Utility/StlMath.h>
 
 #include "Magnum/Math/Dual.h"

src/Magnum/Math/Frustum.h

@@ -30,6 +30,9 @@
  * @brief Class @ref Magnum::Math::Frustum
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #include <Corrade/configure.h>
 #ifndef CORRADE_SINGLES_NO_DEBUG
 #include <Corrade/Utility/Debug.h>

src/Magnum/Math/FunctionsBatch.h

@@ -30,6 +30,9 @@
  */
 
 #include <initializer_list>
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #include <Corrade/Containers/StridedArrayView.h>
 
 #include "Magnum/Math/Functions.h"

src/Magnum/Math/Matrix.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Matrix, alias @ref Magnum::Math::Matrix2x2, @ref Magnum::Math::Matrix3x3, @ref Magnum::Math::Matrix4x4
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/RectangularMatrix.h"
 
 namespace Magnum { namespace Math {

src/Magnum/Math/Matrix3.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Matrix3
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Matrix.h"
 #include "Magnum/Math/Vector3.h"
 

src/Magnum/Math/Matrix4.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Matrix4
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Matrix.h"
 #include "Magnum/Math/Vector4.h"
 

src/Magnum/Math/Quaternion.h

@@ -30,6 +30,9 @@
  * @brief Class @ref Magnum::Math::Quaternion, function @ref Magnum::Math::dot(), @ref Magnum::Math::halfAngle(), @ref Magnum::Math::lerp(), @ref Magnum::Math::slerp()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #ifndef CORRADE_SINGLES_NO_DEBUG
 #include <Corrade/Utility/Debug.h>
 #endif

src/Magnum/Math/Range.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Range, @ref Magnum::Math::Range2D, @ref Magnum::Math::Range3D, alias @ref Magnum::Math::Range1D
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Functions.h"
 #include "Magnum/Math/Vector3.h"
 

src/Magnum/Math/RectangularMatrix.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::RectangularMatrix, alias @ref Magnum::Math::Matrix2x3, @ref Magnum::Math::Matrix3x2, @ref Magnum::Math::Matrix2x4, @ref Magnum::Math::Matrix4x2, @ref Magnum::Math::Matrix3x4, @ref Magnum::Math::Matrix4x3
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Vector.h"
 
 namespace Magnum { namespace Math {

src/Magnum/Math/Vector.h

@@ -29,6 +29,9 @@
  * @brief Class @ref Magnum::Math::Vector, function @ref Magnum::Math::dot(), @ref Magnum::Math::angle()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
 #include <Corrade/Containers/Pair.h>
 #ifndef CORRADE_SINGLES_NO_DEBUG
 #include <Corrade/Utility/Debug.h>

src/Magnum/Math/Vector2.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Vector2, function @ref Magnum::Math::cross()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Vector.h"
 
 namespace Magnum { namespace Math {

src/Magnum/Math/Vector3.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Vector3, function @ref Magnum::Math::cross()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Vector2.h"
 
 namespace Magnum { namespace Math {

src/Magnum/Math/Vector4.h

@@ -29,6 +29,10 @@
  * @brief Class @ref Magnum::Math::Vector4, function @ref Magnum::Math::planeEquation()
  */
 
+/* std::declval() is said to be in <utility> but libstdc++, libc++ and MSVC STL
+   all have it directly in <type_traits> because it just makes sense */
+#include <type_traits>
+
 #include "Magnum/Math/Vector3.h"
 
 namespace Magnum { namespace Math {