"optional" func alias template param

[spir]

On 04/10/2011 04:10 PM, spir wrote:
> Hello,
>
> I need a trick to allow a function template parameter be optional.
> The following (reduced case) fails because D wants to call f:
>
> uint f(uint i) { return i; }
> struct S (alias func=null) {
> enum bool hasFunc = !(func == null); // *** error line ***
> }
> unittest {
> // ok
> auto s1 = S!()();
> writeln(s1.hasFunc);
> // not ok
> auto s2 = S!(f)();
> writeln(s2.hasFunc);
> }
> ==>
> Error: function ___.f (uint i) is not callable using argument types ()
> (I consider that a bug due to "f" implicitely meaning "f()" --but this is
> another story.)
>
> The following also fails:
>
> // not ok
> auto s3 = S1!(&f)();
> writeln(s3.hasFunc);
> ==>
> Error: expression & f is not a valid template value argument

PS: I also cannot pass a typed func:

struct S (uint delegate (uint) func=null) {
     enum bool hasFunc = !(func == null);
}

==>
	Error: arithmetic/string type expected for value-parameter, not uint 
delegate(uint)

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