How to declare a pointer to a template function in D lang? - templates

Question

I'm playing with some D code.
I have some functions filtering on a array:
auto Case1Filters(R)(R lines) if(isInputRange!R) { ... }
auto Case2Filters(R)(R lines) if(isInputRange!R) { ... }
I want to have a function pointer and assign to it one of my filters, so I declared a variable like this:
auto function(R)(R lines) filter = null;
And I get these error when compiling:
conv.d(54): Error: basic type expected, not function
conv.d(54): Error: function declaration without return type. (Note that constructors are always named 'this')
conv.d(54): Error: no identifier for declarator int function(R)(R lines)
conv.d(54): Error: semicolon expected following function declaration
It looks like the compiler thinks that I'm declaring a function.
I tried with an alias, too, but to no avail: is there a way to do this?

Answer 1

You can't have any type of dynamic dispatch to a template, for the same reason you can't have a virtual template function.
Despite all appearances D is a statically typed language which means that all function pointer declarations must have all types available (including the return type auto can only be used when the body is included).
So if R is already declared then you can do:
auto foo(R)(){//R is defined here as some type
Ret function(R lines) filter = null;//no template param when declaring the function pointer and concrete return type
filter = &Case1Filters!R;
filter(r);
}