template auto value

[Jonathan Marler]

I believe I found small hole in template parameter semantics.  
I've summarized it here 
(https://github.com/marler8997/dlangfeatures#template-auto-value-parameter).  Wanted to get feedback before I look into creating a PR for it.

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COPY/PASTED from 
https://github.com/marler8997/dlangfeatures#template-auto-value-parameter
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If you reference the D grammar for templates (See 
https://dlang.org/spec/template.html), there are currently 5 
categories of template parameters:

TemplateParameter:
     TemplateTypeParameter
     TemplateValueParameter
     TemplateAliasParameter
     TemplateSequenceParameter
     TemplateThisParameter
However there is a hole in this list, namely, generic template 
value parameters. The current TemplateValueParameter grammar node 
must explicitly declare a "BasicType":

TemplateValueParameter:
     BasicType Declarator
     BasicType Declarator TemplateValueParameterSpecialization
     BasicType Declarator TemplateValueParameterDefault
     BasicType Declarator TemplateValueParameterSpecialization 
TemplateValueParameterDefault
For example:
---
template foo(string value)
{
}
foo!"hello";
---

However, you can't create a template that accepts a value of any 
type. This would a good use case for the auto keyword, i.e.
---
template foo(auto value)
{
}
foo!0;
foo!"hello";
---

This would be a simple change to the grammar, namely,

BasicTemplateType:
     BasicType
     auto

TemplateValueParameter:
     BasicTemplateType Declarator
     BasicTemplateType Declarator 
TemplateValueParameterSpecialization
     BasicTemplateType Declarator TemplateValueParameterDefault
     BasicTemplateType Declarator 
TemplateValueParameterSpecialization TemplateValueParameterDefault