Math: make Color default alpha values work with Half.

There's some fugliness needed for the MSVC 2017 crash workaround but
otherwise all good.

src/Magnum/Math/Color.h

@@ -37,6 +37,11 @@
 #include <tuple> /** @todo remove when Color[34]::Hsv is removed */
 #endif
 
+#if defined(CORRADE_MSVC2017_COMPATIBILITY) && !defined(CORRADE_MSVC2015_COMPATIBILITY)
+/* Needed by the fullChannel() workaround */
+#include "Magnum/Math/Half.h"
+#endif
+
 namespace Magnum { namespace Math {
 
 namespace Implementation {
@@ -203,15 +208,18 @@ template<class T> inline Vector3<typename Color3<T>::FloatingPointType> toXyz(ty
    projects using CMake. Not using SFINAE in this case makes it work. Minimal
    repro case here: https://twitter.com/czmosra/status/1039446378248896513 */
 template<class T> constexpr typename std::enable_if<IsFloatingPoint<T>::value, T>::type fullChannel() {
-    return T(1);
+    return T(1.0);
 }
 template<class T> constexpr typename std::enable_if<IsIntegral<T>::value, T>::type fullChannel() {
     return Implementation::bitMax<T>();
 }
 #else
 template<class T> constexpr T fullChannel() { return bitMax<T>(); }
-/** @todo half */
 template<> constexpr float fullChannel<float>() { return 1.0f; }
+template<> constexpr Half fullChannel<Half>() {
+    /* This is 1.0_h, but expressible in a constexpr context */
+    return Half{UnsignedShort{0x3c00}};
+}
 template<> constexpr double fullChannel<double>() { return 1.0; }
 template<> constexpr long double fullChannel<long double>() { return 1.0l; }
 #endif

src/Magnum/Math/Test/ColorTest.cpp

@@ -33,6 +33,7 @@
 #endif
 
 #include "Magnum/Math/Color.h"
+#include "Magnum/Math/Half.h"
 #include "Magnum/Math/StrictWeakOrdering.h"
 
 struct Vec3 {
@@ -75,6 +76,7 @@ struct ColorTest: Corrade::TestSuite::Tester {
     explicit ColorTest();
 
     void construct();
+    void constructDefaultAlphaHalf();
     void constructDefault();
     void constructNoInit();
     void constructOneValue();
@@ -152,7 +154,9 @@ typedef Math::Color3<Float> Color3;
 typedef Math::Color3<UnsignedByte> Color3ub;
 
 typedef Math::Vector4<Float> Vector4;
+typedef Math::Vector4<Half> Vector4h;
 typedef Math::Color4<Float> Color4;
+typedef Math::Color4<Half> Color4h;
 typedef Math::Color4<UnsignedByte> Color4ub;
 
 typedef Math::ColorHsv<Float> ColorHsv;
@@ -197,6 +201,7 @@ constexpr struct {
 
 ColorTest::ColorTest() {
     addTests({&ColorTest::construct,
+              &ColorTest::constructDefaultAlphaHalf,
               &ColorTest::constructDefault,
               &ColorTest::constructNoInit,
               &ColorTest::constructOneValue,
@@ -292,6 +297,12 @@ void ColorTest::construct() {
     CORRADE_VERIFY((std::is_nothrow_constructible<Color4, Float, Float, Float, Float>::value));
 }
 
+void ColorTest::constructDefaultAlphaHalf() {
+    /* The default for alpha should work also for the Half type */
+    Color4h a{1.0_h, 0.5_h, 0.75_h};
+    CORRADE_COMPARE(a, Vector4h(1.0_h, 0.5_h, 0.75_h, 1.0_h));
+}
+
 void ColorTest::constructDefault() {
     constexpr Color3 a1;
     constexpr Color3 a2{ZeroInit};