"optional" func alias template param

[spir]

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

Denis
-- 
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