I needed to create a function-like macro which set a parameter in a default settings struct, simple enough.

struct Settings
{
   unsigned enabled;
};

struct Settings defaultSettings;

#define SetEnabled(enabled) (defaultSettings.enabled = enabled)

This compiles fine, and even works in some cases:

// This works fine
bool enabled = true;
SetEnabled(enabled);

I ran into trouble when I renamed the local parameter:

bool newEnabledSetting = true;
SetEnabled(newEnabledSetting);

// Compilation error:
// error: 'struct Settings' has no member named 'newEnabledSetting'

After a bit of head scratching I rechecked the #define and realized the function-like macro parameter and struct parameter shared the same name (enabled).

The #define would look something like this if I send in a parameter named "newEnabledSetting" to it:

bool newEnabledSetting = true;

SetEnabled(newEnabledSetting);
// Is equivalent to
defaultSettings.newEnabledSetting = newEnabledSetting

The lesson for today: make damn sure your function-like macro parameters do not share names with anything you are working with.

#define SetEnabled(macroParamEnabled) (defaultSettings.enabled = macroParamEnabled)