Math: ability to create Dual with ZeroInit tag.

It does slightly different things than the implicit constructor for some
types.

src/Magnum/Math/Dual.h

@@ -59,6 +59,15 @@ template<class T> class Dual {
          */
         constexpr /*implicit*/ Dual() noexcept: _real{}, _dual{} {}
 
+        /** @brief Construct zero-initialized dual number */
+        #ifdef DOXYGEN_GENERATING_OUTPUT
+        constexpr /*implicit*/ Dual(ZeroInitT) noexcept;
+        #else
+        /* MSVC 2015 can't handle {} instead of ::value */
+        template<class U = T, class = typename std::enable_if<std::is_pod<U>::value>::type> constexpr /*implicit*/ Dual(ZeroInitT) noexcept: _real{}, _dual{} {}
+        template<class U = T, class V = T, class = typename std::enable_if<std::is_constructible<U, ZeroInitT>::value>::type> constexpr /*implicit*/ Dual(ZeroInitT) noexcept: _real{ZeroInit}, _dual{ZeroInit} {}
+        #endif
+
         /** @brief Construct without initializing the contents */
         #ifdef DOXYGEN_GENERATING_OUTPUT
         explicit Dual(NoInitT) noexcept;

src/Magnum/Math/Test/DualTest.cpp

@@ -37,6 +37,7 @@ struct DualTest: Corrade::TestSuite::Tester {
 
     void construct();
     void constructDefault();
+    void constructZero();
     void constructNoInit();
     void constructConversion();
     void constructCopy();
@@ -73,6 +74,7 @@ using namespace Literals;
 DualTest::DualTest() {
     addTests({&DualTest::construct,
               &DualTest::constructDefault,
+              &DualTest::constructZero,
               &DualTest::constructNoInit,
               &DualTest::constructConversion,
               &DualTest::constructCopy,
@@ -120,6 +122,16 @@ void DualTest::constructDefault() {
     CORRADE_VERIFY(std::is_nothrow_default_constructible<Dual>::value);
 }
 
+void DualTest::constructZero() {
+    constexpr Dual a{ZeroInit};
+    constexpr Math::Dual<Math::Quaternion<Float>> b{ZeroInit};
+    CORRADE_COMPARE(a, Dual(0.0f, 0.0f));
+    CORRADE_COMPARE(b, Math::Dual<Math::Quaternion<Float>>({{0.0f, 0.0f, 0.0f}, 0.0f}, {{0.0f, 0.0f, 0.0f}, 0.0f}));
+
+    CORRADE_VERIFY((std::is_nothrow_constructible<Dual, ZeroInitT>::value));
+    CORRADE_VERIFY((std::is_nothrow_constructible<Math::Dual<Math::Quaternion<Float>>, ZeroInitT>::value));
+}
+
 void DualTest::constructNoInit() {
     Dual a{2.0f, -7.5f};
     Math::Dual<Math::Quaternion<Float>> b{{{3.0f, 0.1f, 1.0f}, 1.0f}, {{0.1f, 0.0f, 1.0f}, 0.3f}};