auto ref escaping local variable

[Patrick Schluter via Digitalmars-d]

On Tuesday, 24 January 2017 at 06:51:40 UTC, Stefan Koch wrote:
> On Tuesday, 24 January 2017 at 00:52:34 UTC, Robert burner 
> Schadek wrote:
>> I have this program that used to compile with 72 but with 73 
>> dmd is complaining that
>> "Error: escaping reference to local variable t"
>>
>> auto ref f2(T)(auto ref T t, auto ref T s) {
>> 	return t;
>> }
>>
>> auto ref f1(T)(auto ref T t, auto ref T s) {
>> 	return f2(t, s);
>> }
>>
>> unittest {
>> 	int a = f1(1,2);
>> }
>>
>> I'm not sure why, or how to fix that.
>>
>> https://issues.dlang.org/show_bug.cgi?id=17117
>
> Seems to work as expected.
> The literals 1,2 cannot be ref.
> Therefore it's a normal parameter.
> A function parameter is the same as a local
> hence retuning a ref to it will cause this to happen.

Depends on what auto ref is supposed to do, I suppose. What is 
the heuristic used to determine if it should handle a parameter 
as a reference or as a copy?
In the case of f1(1,2) it's obvious, as it is impossible to have 
a reference on the literals, but for f(t, s); both are possible. 
In the case of int and small parameter types sizes a copy would 
be faster.
So the question is, does auto ref use a reference as soon as it 
is possible to have a ref, or should it do so only when it 
"knows" that it is better for performance? I imagine, it's 
actually the first option.
(just thinking loud).