Automatically include the available gl loader header

examples/imgui_impl_opengl3.cpp

@@ -149,6 +149,21 @@ bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
     strcpy(g_GlslVersionString, glsl_version);
     strcat(g_GlslVersionString, "\n");
 
+    // Dummy constructs to make it easily visible in the IDE and debugger which GL loader has been selected.
+    // if you have multiple GL loaders header files accessible to the compiler, the auto detection block in
+    // imgui_impl_opengl3.h may have chosen the wrong one !! 
+    // Use an explicit '#define IMGUI_IMPL_OPENGL_LOADER_XXX' line in imconfig.h or compiler command-line to enforce a certain GL loader.
+    const char* gl_loader = "Unknown";
+    IM_UNUSED(gl_loader);
+#if defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)
+    gl_loader = "GL3W";
+#elif defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)
+    gl_loader = "GLEW";
+#elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
+    gl_loader = "GLAD";
+#elif defined(IMGUI_IMPL_OPENGL_LOADER_CUSTOM)
+    gl_loader = "Custom";
+#endif
     // Make a dummy GL call (we don't actually need the result)
     // IF YOU GET A CRASH HERE: it probably means that you haven't initialized the OpenGL function loader used by this code.
     // Desktop OpenGL 3/4 need a function loader. See the IMGUI_IMPL_OPENGL_LOADER_xxx explanation above.

examples/imgui_impl_opengl3.h

@@ -32,7 +32,38 @@
  && !defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)     \
  && !defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)     \
  && !defined(IMGUI_IMPL_OPENGL_LOADER_CUSTOM)
-#define IMGUI_IMPL_OPENGL_LOADER_GL3W
+    // to avoid problem with non-clang compilers not having this macro.
+    #if defined(__has_include)
+        // check if the header exists, then automatically define the macros ..
+        // check if glew gl loader exists, use it
+        #if __has_include(<GL/glew.h>)
+            // make sure we don't have another gl loader headers ....
+            // that means that the user might have used one and initialized it
+            // while we use the other ....
+            // so we make sure there is only one gl loader header
+            // in this case, we need to only have glew headers and not glad !
+            #if __has_include(<glad/glad.h>)
+                #error you have multiple headers in your system (GLEW + GLAD), remove one of them or use '#define IMGUI_IMPL_OPENGL_LOADER_GLEW'
+            #else
+                #define IMGUI_IMPL_OPENGL_LOADER_GLEW
+            #endif
+        #elif __has_include(<glad/glad.h>)
+            // make sure we don't have another gl loader headers ....
+            // that means that the user might have used one and initialized it
+            // while we use the other ....
+            // so we make sure there is only one gl loader header
+            // in this case, we need to only have glad headers and not glew !
+            #if __has_include(<GL/glew.h>)
+                 #error you have multiple headers in your system (GLEW + GLAD), remove one of them or use '#define IMGUI_IMPL_OPENGL_LOADER_GLAD'
+            #else
+                #define IMGUI_IMPL_OPENGL_LOADER_GLAD
+            #endif
+        #else
+            #define IMGUI_IMPL_OPENGL_LOADER_GL3W
+        #endif
+    #else
+        #define IMGUI_IMPL_OPENGL_LOADER_GL3W
+    #endif
 #endif
 
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_Init(const char* glsl_version = NULL);