Math: test & document why Deg/Rad needs a "copy constructor".

src/Magnum/Math/Angle.h

@@ -124,6 +124,8 @@ template<class T> class Deg: public Unit<Deg, T> {
         template<class U> constexpr explicit Deg(Unit<Math::Deg, U> value) noexcept: Unit<Math::Deg, T>(value) {}
 
         /** @brief Copy constructor */
+        /* Needed in order to make arithmetic operations (which have a Unit
+           return type) convertible to Deg */
         constexpr /*implicit*/ Deg(Unit<Math::Deg, T> other) noexcept: Unit<Math::Deg, T>(other) {}
 
         /**
@@ -211,6 +213,8 @@ template<class T> class Rad: public Unit<Rad, T> {
         template<class U> constexpr explicit Rad(Unit<Math::Rad, U> value) noexcept: Unit<Math::Rad, T>(value) {}
 
         /** @brief Copy constructor */
+        /* Needed in order to make arithmetic operations (which have a Unit
+           return type) convertible to Rad */
         constexpr /*implicit*/ Rad(Unit<Math::Rad, T> value) noexcept: Unit<Math::Rad, T>(value) {}
 
         /**

src/Magnum/Math/Test/AngleTest.cpp

@@ -46,6 +46,7 @@ struct AngleTest: TestSuite::Tester {
     void constructNoInit();
     void constructConversion();
     void constructCopy();
+    void constructFromBase();
 
     void literals();
     void conversion();
@@ -137,6 +138,7 @@ AngleTest::AngleTest() {
               &AngleTest::constructNoInit,
               &AngleTest::constructConversion,
               &AngleTest::constructCopy,
+              &AngleTest::constructFromBase,
 
               &AngleTest::literals,
               &AngleTest::conversion,
@@ -300,6 +302,15 @@ void AngleTest::constructCopy() {
     CORRADE_VERIFY(std::is_nothrow_copy_assignable<Rad>::value);
 }
 
+void AngleTest::constructFromBase() {
+    /* The operation returns Unit instead of the leaf type, so this can work
+       only if the base class has a "copy constructor" from the base type */
+    Deg a = 35.0_degf + 0.15_degf;
+    Radd b = 1.0_rad + 0.25_rad;
+    CORRADE_COMPARE(a, 35.15_degf);
+    CORRADE_COMPARE(b, 1.25_rad);
+}
+
 void AngleTest::literals() {
     constexpr auto a = 25.0_deg;
     CORRADE_VERIFY(std::is_same<decltype(a), const Degd>::value);