Template Class With Default Values

[Jonathan M Davis]

On Sunday, March 04, 2018 11:35:23 bauss via Digitalmars-d-learn wrote:
> Why is the following not working?
>
> class Foo(string baz = "baz")
> {
>      mixin("int " ~ baz ~ ";");
> }
>
> class Bar : Foo
> {
> }
>
> Shouldn't it implicit do, without me having to do it manually?
>
> class Bar : Foo!"baz"
> {
> }
>
> ...
>
> What's the reason you can't specify default values for template
> parameters on a class? Is it intended behavior or is it a bug?
>
> https://run.dlang.io/is/tnOtn3

If it were allowed, it would be ambiguous. e.g. what would

alias Foobar = Foo;

mean? Would it be the default instantiation of Foo!"baz" or would it be the
template itself? Right now, it's always the template itself, and in all
cases except for functions, templates must be explicitly instantiated.

For function templates, we have IFTI (Implicit Function Template
Instantiation) where the compiler tries to infer the template arguments
based on the function arguments. It works because the function call syntax
is unambiguous such that if you have

auto foo(T = string)(T t) {...}

alias foobar = foo;

it's guaranteed that foobar is the template foo and not an instantiation. It
would certainly be possible to make the compiler implicitly instantiate
templates that aren't functions under specific circumstances, but it can't
be done in the general case, so rather than creating a situation with subtle
rules about when it's necessary to explicitly instantiate a template and
when it's not, it's simply always required to explicitly instantiate
templates other than function templates. And based on Walter's comment in
the associated enhancement request, he has concerns that allowing implicit
template instantiations like this would cause serious problems.

https://issues.dlang.org/show_bug.cgi?id=1012

Really, this is one of those things that at first seems like it's crazy that
it doesn't work, but as the details become clearer, it starts seeming a lot
less reasonable to try to make it work.

- Jonathan M Davis