Math: work around bug in GCC 5.1 which broke the tests.

Reported here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 .

src/Magnum/Math/Test/ComplexTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct ComplexConverter<Float, Cmpl> {
-    constexpr static Complex<Float> from(const Cmpl& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static Complex<Float> from(const Cmpl& other) {
         return {other.re, other.im};
     }
 
@@ -164,7 +167,13 @@ void ComplexTest::convert() {
     constexpr Cmpl a{1.5f, -3.5f};
     constexpr Complex b{1.5f, -3.5f};
 
-    constexpr Complex c(a);
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    Complex c(a);
     CORRADE_COMPARE(c, b);
 
     constexpr Cmpl d(b);

src/Magnum/Math/Test/DualComplexTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct DualComplexConverter<Float, DualCmpl> {
-    constexpr static DualComplex<Float> from(const DualCmpl& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static DualComplex<Float> from(const DualCmpl& other) {
         return {{other.re, other.im}, {other.x, other.y}};
     }
 
@@ -158,7 +161,13 @@ void DualComplexTest::convert() {
     constexpr DualCmpl a{1.5f, -3.5f, 7.0f, -0.5f};
     constexpr DualComplex b{{1.5f, -3.5f}, {7.0f, -0.5f}};
 
-    constexpr DualComplex c{a};
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    DualComplex c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr DualCmpl d(b);

src/Magnum/Math/Test/DualQuaternionTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct DualQuaternionConverter<Float, DualQuat> {
-    constexpr static DualQuaternion<Float> from(const DualQuat& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static DualQuaternion<Float> from(const DualQuat& other) {
         return {{{other.re.x, other.re.y, other.re.z}, other.re.w},
                 {{other.du.x, other.du.y, other.du.z}, other.du.w}};
     }
@@ -159,7 +162,13 @@ void DualQuaternionTest::convert() {
     constexpr DualQuat a{{1.5f, -3.5f, 7.0f, -0.5f}, {15.0f, 0.25f, -9.5f, 0.8f}};
     constexpr DualQuaternion b{{{1.5f, -3.5f, 7.0f}, -0.5f}, {{15.0f, 0.25f, -9.5f}, 0.8f}};
 
-    constexpr DualQuaternion c{a};
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    DualQuaternion c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr DualQuat d(b);

src/Magnum/Math/Test/QuaternionTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct QuaternionConverter<Float, Quat> {
-    constexpr static Quaternion<Float> from(const Quat& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static Quaternion<Float> from(const Quat& other) {
         return {{other.x, other.y, other.z}, other.w};
     }
 
@@ -165,7 +168,13 @@ void QuaternionTest::convert() {
     constexpr Quat a{1.5f, -3.5f, 7.0f, -0.5f};
     constexpr Quaternion b{{1.5f, -3.5f, 7.0f}, -0.5f};
 
-    constexpr Quaternion c{a};
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    Quaternion c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr Quat d(b);

src/Magnum/Math/Test/RectangularMatrixTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct RectangularMatrixConverter<2, 3, Float, Mat2x3> {
-    constexpr static RectangularMatrix<2, 3, Float> from(const Mat2x3& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static RectangularMatrix<2, 3, Float> from(const Mat2x3& other) {
         return RectangularMatrix<2, 3, Float>(
             Vector<3, Float>(other.a[0], other.a[1], other.a[2]),
             Vector<3, Float>(other.a[3], other.a[4], other.a[5]));
@@ -211,7 +214,13 @@ void RectangularMatrixTest::convert() {
     constexpr Matrix2x3 b(Vector3(1.5f, 2.0f, -3.5f),
                           Vector3(2.0f, -3.1f,  0.4f));
 
-    constexpr Matrix2x3 c{a};
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    Matrix2x3 c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr Mat2x3 d(b);

src/Magnum/Math/Test/VectorTest.cpp

@@ -38,7 +38,10 @@ namespace Magnum { namespace Math {
 namespace Implementation {
 
 template<> struct VectorConverter<3, Float, Vec3> {
-    constexpr static Vector<3, Float> from(const Vec3& other) {
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr /* See the convert() test case */
+    #endif
+    static Vector<3, Float> from(const Vec3& other) {
         return {other.x, other.y, other.z};
     }
 
@@ -224,7 +227,13 @@ void VectorTest::convert() {
     constexpr Vec3 a{1.5f, 2.0f, -3.5f};
     constexpr Vector3 b(1.5f, 2.0f, -3.5f);
 
-    constexpr Vector3 c{a};
+    /* GCC 5.1 fills the result with zeros instead of properly calling
+       delegated copy constructor if using constexpr. Reported here:
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66450 */
+    #if !defined(__GNUC__) || defined(__clang__)
+    constexpr
+    #endif
+    Vector3 c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr Vec3 d(b);