Some testing?

[bearophile]

Mafi:
> Imagine T was void. Then no return was ok. It wouldn't (shouldn't) 
> compile anyways because a parameter of type void is (should be) invalid. 
> However to enforce the compiler to see that T can't be void is a bit to 
> complicated, I think.

You are right, this different version runs:

T foo(T)() {}
void main() {
    foo!void();
}

Thanks for all the answers, to you, Don and Jonathan.
A question like this was probably more at home in the IRC channel.

Bye,
bearophile