Fix operator[] for vector2d and vector3d being potentially UB (#15977)

We don't have a C++ expert on hand, but taking a pointer to one member
and expecting to access another by an offset is very fishy:
- for one, there could theoretically be padding
- the compiler might assume that we are only writing to that first member

The new code has shown to be free for constant parameter values.
Non-constant ones cause the assembly to have branches (why?), but we don't
use that much.

irr/include/irrTypes.h

@@ -85,6 +85,22 @@ typedef char fschar_t;
 /** prefer to use the override keyword for new code */
 #define _IRR_OVERRIDE_ override
 
+// Invokes undefined behavior for unreachable code optimization
+// Note: an assert(false) is included first to catch this in debug builds
+#if defined(__cpp_lib_unreachable)
+#include <utility>
+#define IRR_CODE_UNREACHABLE() do { _IRR_DEBUG_BREAK_IF(1) std::unreachable(); } while(0)
+#elif defined(__has_builtin)
+#if __has_builtin(__builtin_unreachable)
+#define IRR_CODE_UNREACHABLE() do { _IRR_DEBUG_BREAK_IF(1) __builtin_unreachable(); } while(0)
+#endif
+#elif defined(_MSC_VER)
+#define IRR_CODE_UNREACHABLE() do { _IRR_DEBUG_BREAK_IF(1) __assume(false); } while(0)
+#endif
+#ifndef IRR_CODE_UNREACHABLE
+#define IRR_CODE_UNREACHABLE() (void)0
+#endif
+
 //! creates four CC codes used in Irrlicht for simple ids
 /** some compilers can create those by directly writing the
 code like 'code', but some generate warnings so we use this macro here */

irr/include/vector2d.h

@@ -131,16 +131,20 @@ public:
 
 	T &operator[](u32 index)
 	{
-		_IRR_DEBUG_BREAK_IF(index > 1) // access violation
-
-		return *(&X + index);
+		switch (index) {
+			case 0: return X;
+			case 1: return Y;
+			default: IRR_CODE_UNREACHABLE();
+		}
 	}
 
 	const T &operator[](u32 index) const
 	{
-		_IRR_DEBUG_BREAK_IF(index > 1) // access violation
-
-		return *(&X + index);
+		switch (index) {
+			case 0: return X;
+			case 1: return Y;
+			default: IRR_CODE_UNREACHABLE();
+		}
 	}
 
 	//! sort in order X, Y.

irr/include/vector3d.h

@@ -117,16 +117,22 @@ public:
 
 	T &operator[](u32 index)
 	{
-		_IRR_DEBUG_BREAK_IF(index > 2) // access violation
-
-		return *(&X + index);
+		switch (index) {
+			case 0: return X;
+			case 1: return Y;
+			case 2: return Z;
+			default: IRR_CODE_UNREACHABLE();
+		}
 	}
 
 	const T &operator[](u32 index) const
 	{
-		_IRR_DEBUG_BREAK_IF(index > 2) // access violation
-
-		return *(&X + index);
+		switch (index) {
+			case 0: return X;
+			case 1: return Y;
+			case 2: return Z;
+			default: IRR_CODE_UNREACHABLE();
+		}
 	}
 
 	//! sort in order X, Y, Z.

irr/src/OpenGL/Driver.cpp

@@ -104,8 +104,7 @@ static const VertexType &getVertexTypeDescription(E_VERTEX_TYPE type)
 	case EVT_TANGENTS:
 		return vtTangents;
 	default:
-		assert(false);
-		CODE_UNREACHABLE();
+		IRR_CODE_UNREACHABLE();
 	}
 }
 

irr/src/os.h

@@ -10,22 +10,6 @@
 #include "ILogger.h"
 #include "ITimer.h"
 
-// CODE_UNREACHABLE(): Invokes undefined behavior for unreachable code optimization
-#if defined(__cpp_lib_unreachable)
-#include <utility>
-#define CODE_UNREACHABLE() std::unreachable()
-#elif defined(__has_builtin)
-#if __has_builtin(__builtin_unreachable)
-#define CODE_UNREACHABLE() __builtin_unreachable()
-#endif
-#elif defined(_MSC_VER)
-#define CODE_UNREACHABLE() __assume(false)
-#endif
-
-#ifndef CODE_UNREACHABLE
-#define CODE_UNREACHABLE() (void)0
-#endif
-
 namespace irr
 {