Template argument deduction and default args

[Manu via Digitalmars-d]

On 24 July 2014 21:30, Manu <turkeyman at gmail.com> wrote:

> On 24 July 2014 21:25, Manu <turkeyman at gmail.com> wrote:
>
>> On 24 July 2014 19:37, John Colvin via Digitalmars-d <
>> digitalmars-d at puremagic.com> wrote:
>>
>>> On Thursday, 24 July 2014 at 04:53:41 UTC, Manu via Digitalmars-d wrote:
>>>
>>>> I'm running into consistent problems with default args and argument
>>>> deduction in templates.
>>>> There seem to be 2 consistent classes of problem:
>>>>
>>>> struct S(size_t len = 10)
>>>> {
>>>>   ubyte[len] data;
>>>> }
>>>>
>>>> S!100 x; // this works fine
>>>> S y; // this doesn't work (!)
>>>> S!() z; // this works
>>>>
>>>> The template arg has a default arg, why require !() ??
>>>> This causes problems in meta code, where you want to create an instance
>>>> of
>>>> some T, and T may be a normal type with no template args, in which case
>>>> !()
>>>> is invalid, but a template type with default args should also be
>>>> acceptable, but it doesn't work because the meta code doesn't specify
>>>> !().
>>>>
>>>
>>> This opens a whole can of worms. It's very useful to be able to
>>> distinguish between templates and their instantiations. Seeing as D's alias
>>> system works on a pass-by-name system, you can't have a system where simply
>>> referring to a template instantiates it with no arguments.
>>>
>>> Apart from anything else it would break *so* much code.
>>>
>>
>> Isn't the call to the constructor enough to distinguish it is an
>> instantiation rather than a reference to the template itself?
>>
>> S is a template
>> S!() is a type
>> S(x,y,z) is a call to it's constructor, the expression has a type
>>
>>  What is the useful distinction between S!()(x,y,z) and S(x,y,z)? How
>> does either one of them make referring to the template 'S' difficult?
>> Is there some conflicting syntax where the parentheses mean something
>> else when S perceived as a template? Why isn't the same problem applicable
>> to function templates?
>>
>
> Ack! Sorry! I misread, your response as being related to the constructor
> case, not the default arg case >_<
>
> I see the problem with the default arg case. It's a real shame, because it
> has rather annoying side effects in generic code, and it doesn't appear to
> follow the same logical rules as with functions.
> This is precisely the sort of thing Scott Myers would be unhappy about...
> Someone will need to 'explain' this for years to come, I don't think it's
> intuitive ;)
>

Although... the more I think about it, the more I wonder why it matters if
the syntax is ambiguous when the name is taken in isolation, that never
actually happens...
Why can't it just mean 'either the template, or the default arg
instantiation', and be resolved when it's actually used?
Is it possible for templates or types to both appear in the same context
and create an actual ambiguity? What would that expression look like?
The only place I can imagine a conflict could occur would be within an is()
expression, but I'm not sure... can a uninstantiated template be used in an
is() expression where a type would also be a meaningful fit?

Generally, templates do this:
  T!()
And types do this:
  T var;

It's clear syntactically from 'T!()' that T is not a default args
instantiation of T, because it's involved in a template instantiation
expression.
It's also clear from 'T var' that T is not a template, because a variable
needs to have a type.
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