Compile-time checks to ensure correct behavior for pointer arithmetic

imgui.h

@@ -43,7 +43,7 @@ Index of this file:
 // [SECTION] Dear ImGui end-user API functions
 // [SECTION] Flags & Enumerations
 // [SECTION] Tables API flags and structures (ImGuiTableFlags, ImGuiTableColumnFlags, ImGuiTableRowFlags, ImGuiTableBgTarget, ImGuiTableSortSpecs, ImGuiTableColumnSortSpecs)
-// [SECTION] Helpers: Debug log, Memory allocations macros, ImVector<>
+// [SECTION] Helpers: Debug log, Memory allocations macros, ImVector<>, Unreachable
 // [SECTION] ImGuiStyle
 // [SECTION] ImGuiIO
 // [SECTION] Misc data structures (ImGuiInputTextCallbackData, ImGuiSizeCallbackData, ImGuiPayload)
@@ -287,6 +287,7 @@ typedef int     (*ImGuiInputTextCallback)(ImGuiInputTextCallbackData* data);
 typedef void    (*ImGuiSizeCallback)(ImGuiSizeCallbackData* data);              // Callback function for ImGui::SetNextWindowSizeConstraints()
 typedef void*   (*ImGuiMemAllocFunc)(size_t sz, void* user_data);               // Function signature for ImGui::SetAllocatorFunctions()
 typedef void    (*ImGuiMemFreeFunc)(void* ptr, void* user_data);                // Function signature for ImGui::SetAllocatorFunctions()
+namespace ImGui { [[noreturn]] inline void Unreachable(); };                    // Function signature for ImGui::Unreachable()
 
 // ImVec2: 2D vector used to store positions, sizes etc. [Compile-time configurable type]
 // - This is a frequently used type in the API. Consider using IM_VEC2_CLASS_EXTRA to create implicit cast from/to our preferred type.
@@ -297,8 +298,15 @@ struct ImVec2
     float                                   x, y;
     constexpr ImVec2()                      : x(0.0f), y(0.0f) { }
     constexpr ImVec2(float _x, float _y)    : x(_x), y(_y) { }
-    float& operator[] (size_t idx)          { IM_ASSERT(idx == 0 || idx == 1); return ((float*)(void*)(char*)this)[idx]; } // We very rarely use this [] operator, so the assert overhead is fine.
-    float  operator[] (size_t idx) const    { IM_ASSERT(idx == 0 || idx == 1); return ((const float*)(const void*)(const char*)this)[idx]; }
+    #define SUBSCRIPT_OPERATION \
+        switch (idx % 2) {      \
+            case 0: return x;   \
+            case 1: return y;   \
+            default: ImGui::Unreachable();  /* Compiler hint: ensures optimization is taken and removes a false positive non-returning branch. */ \
+        };
+    float& operator[] (size_t idx) { SUBSCRIPT_OPERATION; }
+    float operator[] (size_t idx) const { SUBSCRIPT_OPERATION; }
+    #undef SUBSCRIPT_OPERATION
 #ifdef IM_VEC2_CLASS_EXTRA
     IM_VEC2_CLASS_EXTRA     // Define additional constructors and implicit cast operators in imconfig.h to convert back and forth between your math types and ImVec2.
 #endif
@@ -2150,7 +2158,7 @@ struct ImGuiTableColumnSortSpecs
 };
 
 //-----------------------------------------------------------------------------
-// [SECTION] Helpers: Debug log, memory allocations macros, ImVector<>
+// [SECTION] Helpers: Debug log, memory allocations macros, ImVector<>, Unreachable
 //-----------------------------------------------------------------------------
 
 //-----------------------------------------------------------------------------
@@ -2255,6 +2263,37 @@ struct ImVector
 };
 IM_MSVC_RUNTIME_CHECKS_RESTORE
 
+//-----------------------------------------------------------------------------
+// ImGui::Unreachable()
+// Provides an equivalent for std::unreachable() for C++<23. This signals to the compiler that a region is unreachable, allowing optimizations.
+//-----------------------------------------------------------------------------
+// If you are using C++>=23, use std::unreachable() in <utility>, and you may also define STD_UNREACHABLE_IS_AVAILABLE to substitute definitions.
+// Code is from: https://en.cppreference.com/w/cpp/utility/unreachable.html
+// Inclusion in the ImGUI namespace is chosen to avoid possible namespace collisions.
+//-----------------------------------------------------------------------------
+namespace ImGui
+{
+//#define STD_UNREACHABLE_IS_AVAILABLE
+#ifdef STD_UNREACHABLE_IS_AVAILABLE 
+    #include <utility>
+    constexpr const auto Unreachable = ::std::unreachable;
+    #undef STD_UNREACHABLE_IS_AVAILABLE
+#else
+    [[noreturn]] inline void Unreachable()
+    {
+        // Uses compiler specific extensions if possible.
+        // Even if no extension is used, undefined behavior is still raised by
+        // an empty function body and the noreturn attribute.
+    #if defined(_MSC_VER) && !defined(__clang__) // MSVC
+        __assume(false);
+    #else // GCC, Clang
+        __builtin_unreachable();
+    #endif
+    }
+#endif
+
+} // namespace ImGui
+
 //-----------------------------------------------------------------------------
 // [SECTION] ImGuiStyle
 //-----------------------------------------------------------------------------