Bug 599 – ExpressionTuple doesn't match template's alias parameters

template test2(alias s){ alias s[0] test2; } void test(T...)(T tl){ alias tl tl2;//ok assert(test2!(tl)==0);//doesn't match alias template declaration } void main(){ test(0,"a",'a'); }

Here's what I get (DMD 0.121, Windows): bz599.d(7): template instance test2!(_param_0,_param_1,_param_2) does not match any template declaration bz599.d(7): Error: void has no value bz599.d(7): Error: incompatible types for ((test2!(_param_0,_param_1,_param_2)) == (0)): 'void' and 'int' bz599.d(11): template instance bz599.test!(int,char[1u],char) error instantiating The problem seems to be that it doesn't like a tuple as a template alias parameter. From what I can make out of the spec, it ought to work.

On second thoughts, it seems the problem is that test2!(t1) expands to test2!(0, "a", 'a'), which is three parameters, whereas (alias s) is only one. Still, it ought to be possible to pass tuples as alias parameters.

Tuples are expanded before arguments are matched to parameters, so they cannot be passed as alias parameters. This is by design.

If you need to allow someone to do this for your template, try this: struct Tpack(T...) { static alias T Tpl; } test2!(Tpack!(T)) template test2(alias s){ alias s.Tpl[0] test2; } (Or some variate of that, I didn't test this but have made the trick work before)