[DIP] Resolution of Alias Template Parameters in Function Templates

[Stefanos Baziotis]

On Monday, 20 May 2019 at 19:07:49 UTC, jmh530 wrote:
> On Monday, 20 May 2019 at 19:05:06 UTC, jmh530 wrote:
>> On Monday, 20 May 2019 at 18:20:06 UTC, Stefanos Baziotis 
>> wrote:
>>> [snip]
>>>
>>> But with the above reasoning we want this to fail (that is, 
>>> evaluate is(T == int) as true and go there) because aliases 
>>> and template aliases are indistinguishable from
>>> the type they alias.
>>
>> Sorry, it would be true for Num!int, but false for Num!float 
>> or something else.
>
> It makes more sense if I said it should evaluate is(T==int) in 
> the first static if for Num!int and then go to the second 
> static if and evaluate is(T==float) for Num!float.
>
> That's just because of the way you have the static ifs set up.

If the second static if is `if(is(T == float))` and we passed 
float or Num!float,
yes it should go there. If it is as what we said earlier, `is(T : 
Num!U, U)` it should not evaluate it to true with the reasoning I 
said above. That is, it doesn't distinguish an int from a Num!int 
or a float from a Num!float. It goes back to the assumption that 
aliases and template aliases are indistinguishable from the type 
they alias.

Again, that is what I _assume_ that is the general direction. The 
DIP is a step towards that (basically, the root of the problem 
that the DIP solves is that
the compiler sees the alias and the actual type as 2 different 
types). Also, that is
what _I_ would think as the better alternative of the 2. That is 
not to say
that it is the correct / best one.

Certainly, it's what the compiler source seems to move towards. 
During the semantic analysis, the type of a Num!int variable, 
becomes an int. I surely don't
have a lot of experience with the compiler internals, but as far 
as I know, there's
no clear / easy way to know whether it came from an alias or an 
int.