If you put a function inside #ifdef --- #endif block which technically makes her disabled, code-wise, editor still "thinks" it's active:
1) Typing its name will make a tip appear showing its full name and parameters.
2) Right clicking on the typed function call and selecting "Goto Definition" will make editor go to disabled function's body.
3) If the function was used as an event handler and is still typed into object's properties pane, double clicking on its name there makes editor go to function definition.
If you make second function with same name, and make it active, "go to definition" still goes to previous one.
The auto-complete is aware of a couple of #defines (STRICT, STRICT_STRINGS, STRICT_AUDIO), but it currently doesn't parse custom #defines.
This is such a rarely-used feature, that I'm not sure adding support for it can be justified.