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f(ByRef p:=""){
	if not p	;endless loop here
		p:=0
	if(p++<2)
		f(p)
	return p
}
MsgBox % f()

Although a function may call itself recursively, if it passes one of its own local variables or non-ByRef parameters to itself ByRef, the new layer's ByRef parameter will refer to its own local variable of that name rather than the previous layer's. However, this issue does not occur when a function passes to itself a global variable, static variable, or ByRef parameter.

[v1.0.46.13+]: ByRef parameters also support default values; for example: MyFunc(ByRef p1 = ""). Whenever the caller omits such a parameter, the function creates a local variable to contain the default value; in other words, the function behaves as though the keyword "ByRef" is absent.