Return by reference

[Brian Rogoff <bpr@shell5.ba.best.com>]

I understand why, for a return-by-reference type T, the function 

function Some_Func( ... ) return T is 
    Local_Var : T
begin
    ...
    return Local_Var;
end Some_Func;

causes Program_Error to be raised at run time, but I'm a bit 
perplexed as to why there is a problem for 

function Some_Func( Param : T; ... ) return T is
begin
    ...
    return Param;
end Some_Func;

Could someone explain why the latter case is problematic? 

-- Brian