Default Template Instantiation

[Jonathan Marler via Digitalmars-d]

On Monday, 19 September 2016 at 19:53:27 UTC, Basile B. wrote:
> On Monday, 19 September 2016 at 19:38:37 UTC, Jonathan Marler 
> wrote:
>> If you have a template where:
>>
>> 1) All parameters are optional
>> 2) The parameters cannot be deduced
>>
>> Would it be reasonable to instantiate the template with the 
>> default parameter values? For example:
>>
>> template Foo(string str = "some string", T...)
>> {
>>     class Foo
>>     {
>>         // class implementation
>>         this() { }
>>     }
>> }
>>
>> auto foo = new Foo!()(); // OK
>> auto foo = new Foo(); // Error: cannot deduce template 
>> parameters
>>
>> In this example, there's no way to deduce the template 
>> parameters for Foo, however, since they are not specified, 
>> would it be reasonable to just use the defaults?  str would be 
>> "some string" and T would be empty?
>
> My understanding of this case is that the usage of the 
> eponymous member must include all the template parameters 
> because for example we could have:
>
> template Foo(string str = "some string", T...)
> {
>     class Foo
>     {
>         T[0] a;
>         T[1] b;
>         // class implementation
>         this() { }
>     }
> }
>
> The way "new Foo" is used must tells what are the parent 
> template parameters.

Yes that's why the template cannot deduce the parameters. The 
question is, when the parameters cannot be deduced, and they are 
all optional, would it be reasonable for the compiler to infer 
that the user intended to use the default parameters?