Don't rely on __GNUC__ being always 4 on Clang.

Certain Clang-based IDEs (CLion) "emulate" a compiler by inheriting all
its defines, which means one gets __clang__ defined but also __GNUC__
set to 11 or whatever, breaking all these assumptions.

src/Magnum/Math/Test/AngleTest.cpp

@@ -188,7 +188,10 @@ void AngleTest::constructNoInit() {
     new(&a) Deg{Magnum::NoInit};
     new(&b) Rad{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601
         /* The warning is reported for both debug and release build */
         #pragma GCC diagnostic push
         #pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
@@ -198,7 +201,7 @@ void AngleTest::constructNoInit() {
         #endif
         CORRADE_COMPARE(Float(a), 25.0f);
         CORRADE_COMPARE(Float(b), 3.14f);
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601
         #pragma GCC diagnostic pop
         #endif
     }

src/Magnum/Math/Test/BezierTest.cpp

@@ -142,7 +142,10 @@ void BezierTest::constructNoInit() {
     QuadraticBezier2D a{Vector2{0.5f, 1.0f}, Vector2{1.1f, 0.3f}, Vector2{0.1f, 1.2f}};
     new(&a) QuadraticBezier2D{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (QuadraticBezier2D{Vector2{0.5f, 1.0f}, Vector2{1.1f, 0.3f}, Vector2{0.1f, 1.2f}}));

src/Magnum/Math/Test/BoolVectorTest.cpp

@@ -139,7 +139,10 @@ void BoolVectorTest::constructNoInit() {
     BoolVector19 a{0xa5, 0x5f, 0x07};
     new(&a) BoolVector19{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, BoolVector19(0xa5, 0x5f, 0x07));

src/Magnum/Math/Test/ColorTest.cpp

@@ -336,7 +336,10 @@ void ColorTest::constructNoInit() {
     new(&a) Color3{Magnum::NoInit};
     new(&b) Color4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Color3{1.0f, 0.5f, 0.75f}));
@@ -518,7 +521,10 @@ void ColorTest::constructHsvNoInit() {
     ColorHsv a{135.0_degf, 0.5f, 0.9f};
     new(&a) ColorHsv{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (ColorHsv{135.0_degf, 0.5f, 0.9f}));

src/Magnum/Math/Test/ComplexTest.cpp

@@ -207,7 +207,10 @@ void ComplexTest::constructNoInit() {
     Complex a{0.5f, -3.7f};
     new(&a) Complex{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Complex(0.5f, -3.7f));

src/Magnum/Math/Test/CubicHermiteTest.cpp

@@ -434,7 +434,10 @@ void CubicHermiteTest::constructNoInitScalar() {
     CubicHermite1D spline{2.0f, -2.0f, -0.5f};
     new(&spline) CubicHermite1D{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(spline, (CubicHermite1D{2.0f, -2.0f, -0.5f}));
@@ -450,7 +453,10 @@ void CubicHermiteTest::constructNoInitVector() {
     CubicHermite2D spline{{1.0f, 2.0f}, {1.5f, -2.0f}, {3.0f, -0.5f}};
     new(&spline) CubicHermite2D{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(spline, (CubicHermite2D{{1.0f, 2.0f}, {1.5f, -2.0f}, {3.0f, -0.5f}}));
@@ -466,7 +472,10 @@ void CubicHermiteTest::constructNoInitComplex() {
     CubicHermiteComplex spline{{1.0f, 2.0f}, {1.5f, -2.0f}, {3.0f, -0.5f}};
     new(&spline) CubicHermiteComplex{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(spline, (CubicHermiteComplex{{1.0f, 2.0f}, {1.5f, -2.0f}, {3.0f, -0.5f}}));
@@ -485,7 +494,10 @@ void CubicHermiteTest::constructNoInitQuaternion() {
         {{3.0f, -0.5f, 1.2f}, 0.3f}};
     new(&spline) CubicHermiteQuaternion{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(spline, (CubicHermiteQuaternion{

src/Magnum/Math/Test/DualComplexTest.cpp

@@ -203,7 +203,10 @@ void DualComplexTest::constructNoInit() {
     DualComplex a{{-1.0f, 2.5f}, {3.0f, -7.5f}};
     new(&a) DualComplex{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, DualComplex({-1.0f, 2.5f}, {3.0f, -7.5f}));

src/Magnum/Math/Test/DualQuaternionTest.cpp

@@ -232,7 +232,10 @@ void DualQuaternionTest::constructNoInit() {
     DualQuaternion a{{{1.0f, 2.0f, 3.0f}, -4.0f}, {{0.5f, -3.1f, 3.3f}, 2.0f}};
     new(&a) DualQuaternion{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, DualQuaternion({{1.0f, 2.0f, 3.0f}, -4.0f}, {{0.5f, -3.1f, 3.3f}, 2.0f}));

src/Magnum/Math/Test/DualTest.cpp

@@ -151,7 +151,10 @@ void DualTest::constructNoInit() {
     new(&a) Dual{Magnum::NoInit};
     new(&b) Math::Dual<Math::Quaternion<Float>>{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Dual(2.0f, -7.5f));

src/Magnum/Math/Test/FrustumTest.cpp

@@ -192,7 +192,10 @@ void FrustumTest::constructNoInit() {
 
     new(&a) Frustum{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
 
@@ -311,7 +314,7 @@ void FrustumTest::data() {
     /* Using default-constructed to verify that the planes are in correct order */
     constexpr Frustum a;
 
-    #if !defined(__GNUC__) || __GNUC__*100 + __GNUC_MINOR__ >= 500
+    #if !defined(CORRADE_TARGET_GCC) || defined(CORRADE_TARGET_CLANG) || __GNUC__ >= 5
     constexpr
     #endif
     Vector4 right = a.cbegin()[1];
@@ -323,7 +326,7 @@ void FrustumTest::data() {
     constexpr Vector4 near = a.near();
     CORRADE_COMPARE(near, (Vector4{0.0f, 0.0f, 1.0f, 1.0f}));
 
-    #if !defined(CORRADE_MSVC2015_COMPATIBILITY) && (!defined(__GNUC__) || __GNUC__*100 + __GNUC_MINOR__ >= 500)
+    #if !defined(CORRADE_MSVC2015_COMPATIBILITY) && (!defined(CORRADE_TARGET_GCC) || defined(CORRADE_TARGET_CLANG) || __GNUC__ >= 5)
     constexpr
     #endif
     Vector4 far = *(a.cend() - 1);

src/Magnum/Math/Test/HalfTest.cpp

@@ -544,7 +544,10 @@ void HalfTest::constructNoInit() {
     Half a{3.5f};
     new(&a) Half{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Half{3.5f});

src/Magnum/Math/Test/Matrix3Test.cpp

@@ -214,7 +214,10 @@ void Matrix3Test::constructNoInit() {
               {7.9f, -1.0f, 8.0f}};
     new(&a) Matrix3{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Matrix3({3.0f,  5.0f, 8.0f},

src/Magnum/Math/Test/Matrix4Test.cpp

@@ -254,7 +254,10 @@ void Matrix4Test::constructNoInit() {
                  {7.9f, -1.0f, 8.0f, -1.5f}};
     new(&a) Matrix4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Matrix4({3.0f,  5.0f, 8.0f, -3.0f},

src/Magnum/Math/Test/MatrixTest.cpp

@@ -190,7 +190,10 @@ void MatrixTest::constructNoInit() {
                 Vector4(7.9f, -1.0f, 8.0f, -1.5f)};
     new(&a) Matrix4x4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Matrix4x4(Vector4(3.0f,  5.0f, 8.0f, -3.0f),

src/Magnum/Math/Test/QuaternionTest.cpp

@@ -250,7 +250,10 @@ void QuaternionTest::constructNoInit() {
     Quaternion a{{1.0f, 2.0f, 3.0f}, -4.0f};
     new(&a) Quaternion{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Quaternion({1.0f, 2.0f, 3.0f}, -4.0f));

src/Magnum/Math/Test/RangeTest.cpp

@@ -262,7 +262,10 @@ void RangeTest::constructNoInit() {
     new(&c) Range3Di{Magnum::NoInit};
 
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Range1Di{3, 23}));

src/Magnum/Math/Test/RectangularMatrixTest.cpp

@@ -215,7 +215,10 @@ void RectangularMatrixTest::constructNoInit() {
                 Vector4(9.0f, 10.0f, 11.0f, 12.0f)};
     new(&a) Matrix3x4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Matrix3x4(Vector4(1.0f,  2.0f,  3.0f,  4.0f),

src/Magnum/Math/Test/UnitTest.cpp

@@ -101,7 +101,10 @@ void UnitTest::constructNoInit() {
     Sec a{25.0f};
     new(&a) Sec{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, Sec{25.0f});

src/Magnum/Math/Test/Vector2Test.cpp

@@ -128,7 +128,10 @@ void Vector2Test::constructNoInit() {
     Vector2 a{1.5f, 2.5f};
     new(&a) Vector2{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Vector2{1.5f, 2.5f}));

src/Magnum/Math/Test/Vector3Test.cpp

@@ -128,7 +128,10 @@ void Vector3Test::constructNoInit() {
     Vector3 a{1.0f, 2.5f, -3.0f};
     new(&a) Vector3{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Vector3{1.0f, 2.5f, -3.0f}));

src/Magnum/Math/Test/Vector4Test.cpp

@@ -149,7 +149,10 @@ void Vector4Test::constructNoInit() {
     Vector4 a{1.0f, -2.5f, 3.0f, 4.1f};
     new(&a) Vector4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Vector4{1.0f, -2.5f, 3.0f, 4.1f}));

src/Magnum/Math/Test/VectorTest.cpp

@@ -244,7 +244,10 @@ void VectorTest::constructNoInit() {
     Vector4 a{1.0f, 2.0f, -3.0f, 4.5f};
     new(&a) Vector4{Magnum::NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a, (Vector4{1.0f, 2.0f, -3.0f, 4.5f}));

src/Magnum/Platform/Implementation/DpiScaling.cpp

@@ -157,13 +157,17 @@ bool isWindowsAppDpiAware() {
     if(shcore) {
         /* GCC 8 adds -Wcast-function-type, enabled by default with -Wextra,
            which causes this line to emit a warning on MinGW. We know what
-           we're doing, so suppress that. */
-        #if defined(CORRADE_TARGET_GCC) && __GNUC__ >= 8
+           we're doing, so suppress that.
+
+           Also explicitly check we're not on Clang because certain Clang-based
+           IDEs inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does. */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__ >= 8
         #pragma GCC diagnostic push
         #pragma GCC diagnostic ignored "-Wcast-function-type"
         #endif
         auto* const getProcessDpiAwareness = reinterpret_cast<HRESULT(WINAPI *)(HANDLE, PROCESS_DPI_AWARENESS*)>(GetProcAddress(shcore, "GetProcessDpiAwareness"));
-        #if defined(CORRADE_TARGET_GCC) && __GNUC__ >= 8
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__ >= 8
         #pragma GCC diagnostic pop
         #endif
         PROCESS_DPI_AWARENESS result{};
@@ -178,13 +182,17 @@ bool isWindowsAppDpiAware() {
     CORRADE_INTERNAL_ASSERT(user32);
     /* GCC 8 adds -Wcast-function-type, enabled by default with -Wextra, which
        causes this line to emit a warning on MinGW. We know what we're doing,
-       so suppress that. */
-    #if defined(CORRADE_TARGET_GCC) && __GNUC__ >= 8
+       so suppress that.
+
+       Also explicitly check we're not on Clang because certain Clang-based
+       IDEs inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+       Clang itself does. */
+    #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__ >= 8
     #pragma GCC diagnostic push
     #pragma GCC diagnostic ignored "-Wcast-function-type"
     #endif
     auto const isProcessDPIAware = reinterpret_cast<BOOL(WINAPI *)()>(GetProcAddress(user32, "IsProcessDPIAware"));
-    #if defined(CORRADE_TARGET_GCC) && __GNUC__ >= 8
+    #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__ >= 8
     #pragma GCC diagnostic pop
     #endif
     CORRADE_INTERNAL_ASSERT(isProcessDPIAware);

src/Magnum/Shaders/Test/DistanceFieldVectorTest.cpp

@@ -108,7 +108,10 @@ void DistanceFieldVectorTest::drawUniformConstructNoInit() {
 
     new(&a) DistanceFieldVectorDrawUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.materialId, 76);
@@ -176,7 +179,10 @@ void DistanceFieldVectorTest::materialUniformConstructNoInit() {
 
     new(&a) DistanceFieldVectorMaterialUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.color, 0x354565fc_rgbaf);

src/Magnum/Shaders/Test/FlatTest.cpp

@@ -113,7 +113,10 @@ void FlatTest::drawUniformConstructNoInit() {
 
     new(&a) FlatDrawUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.materialId, 5);
@@ -172,7 +175,10 @@ void FlatTest::materialUniformConstructNoInit() {
 
     new(&a) FlatMaterialUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.color, 0x354565fc_rgbaf);

src/Magnum/Shaders/Test/GenericTest.cpp

@@ -157,7 +157,10 @@ void GenericTest::projectionUniform2DConstructNoInit() {
 
     new(&a) ProjectionUniform2D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.projectionMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -223,7 +226,10 @@ void GenericTest::projectionUniform3DConstructNoInit() {
 
     new(&a) ProjectionUniform3D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.projectionMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -281,7 +287,10 @@ void GenericTest::transformationUniform2DConstructNoInit() {
 
     new(&a) TransformationUniform2D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.transformationMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -347,7 +356,10 @@ void GenericTest::transformationUniform3DConstructNoInit() {
 
     new(&a) TransformationUniform3D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.transformationMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -411,7 +423,10 @@ void GenericTest::textureTransformationUniformConstructNoInit() {
 
     new(&a) TextureTransformationUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.rotationScaling[1], (Vector2{2.5f, -3.0f}));

src/Magnum/Shaders/Test/MeshVisualizerTest.cpp

@@ -123,7 +123,10 @@ void MeshVisualizerTest::drawUniform2DConstructNoInit() {
 
     new(&a) MeshVisualizerDrawUniform2D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.materialId, 73);
@@ -195,7 +198,10 @@ void MeshVisualizerTest::drawUniform3DConstructNoInit() {
 
     new(&a) MeshVisualizerDrawUniform3D{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.normalMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -282,7 +288,10 @@ void MeshVisualizerTest::materialUniformConstructNoInit() {
 
     new(&a) MeshVisualizerMaterialUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.color, 0x354565fc_rgbaf);

src/Magnum/Shaders/Test/PhongTest.cpp

@@ -155,7 +155,10 @@ void PhongTest::drawUniformConstructNoInit() {
 
     new(&a) PhongDrawUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.normalMatrix[2], (Vector4{1.5f, 0.3f, 3.1f, 0.5f}));
@@ -241,7 +244,10 @@ void PhongTest::materialUniformConstructNoInit() {
 
     new(&a) PhongMaterialUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.diffuseColor, 0x354565fc_rgbaf);
@@ -309,7 +315,10 @@ void PhongTest::lightUniformConstructNoInit() {
 
     new(&a) PhongLightUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.color, 0x354565_rgbf);

src/Magnum/Shaders/Test/VectorTest.cpp

@@ -108,7 +108,10 @@ void VectorTest::drawUniformConstructNoInit() {
 
     new(&a) VectorDrawUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.materialId, 5);
@@ -164,7 +167,10 @@ void VectorTest::materialUniformConstructNoInit() {
 
     new(&a) VectorMaterialUniform{NoInit};
     {
-        #if defined(__GNUC__) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
+        /* Explicitly check we're not on Clang because certain Clang-based IDEs
+           inherit __GNUC__ if GCC is used instead of leaving it at 4 like
+           Clang itself does */
+        #if defined(CORRADE_TARGET_GCC) && !defined(CORRADE_TARGET_CLANG) && __GNUC__*100 + __GNUC_MINOR__ >= 601 && __OPTIMIZE__
         CORRADE_EXPECT_FAIL("GCC 6.1+ misoptimizes and overwrites the value.");
         #endif
         CORRADE_COMPARE(a.color, 0x354565fc_rgbaf);