Bug 9864 – Allow instantiating eponymous inner template with a single parameter list

Here's a hasField template which allows you to instantiate it with only one parameter to create an alias: ----- template hasField(string name) { template hasField(T) { enum bool hasField = __traits(hasMember, T, name); } } struct S1 { int x; } struct S2 { int y; } void main() { alias hasX = hasField!"x"; static assert(hasX!S1); static assert(!hasX!S2); static assert(hasField!("x", S1)); // ng } ----- Unfortunately we cannot use this template directly. We can turn it into a normal template: ----- template hasField(string name, T) { enum bool hasField = __traits(hasMember, T, name); } ----- But then we lose the ability to alias the template with a single type parameter: ----- alias hasX = hasField!"x"; // ng ----- There is a library workaround for this, by introducing a Curry template: ----- template Curry(alias Templ, T...) { template Curry(X...) { alias Curry = Templ!(T, X); } } template hasField(string name, T) { enum bool hasField = __traits(hasMember, T, name); } struct S1 { int x; } struct S2 { int y; } void main() { alias hasX = Curry!(hasField, "x"); // ok static assert(hasX!S1); static assert(!hasX!S2); static assert(hasField!("x", S1)); // ok } ----- Nevertheless it would be nice to be able to use the first syntax as well. I consider this a low priority enhancement since a library workaround exists.

Needs to be a DIP, but also this would be a big change in language semantics. I don't really need it as much as I thought I did.