On Wednesday, 24 January 2018 at 07:21:09 UTC, Shachar Shemesh
wrote:
test.d(6): Error: struct test.A(int var = 3) is used as a type
Of course it is. That's how structs are used.
Program causing this:
struct A(int var = 3) {
int a;
}
void main() {
A a;
}
To resolve, you need to change A into A!(). For some reason I
have not been able to fathom, default template parameters on
structs don't work like they do on functions.
IMO the error message is not too bad once you understand what's
going on (which probably means it's really not a good error
message).
struct A(int var = 3) is short for:
template A(int var = 3)
{
struct A
{
//...
}
}
As I'm sure you know.
If it's written out like this then I think it makes it obvious
what the problem is. When you write `A a`, you're trying to use
this template like a type, but templates are not types; they are
used to _construct_ types. Therefore, to construct a valid type
from the template A, you have to instantiate it by declaring an
`A!() a`.
The compiler could allow implicit insatntiation if the template
has 0 arguments or only default arguments, but as Jonathan
showed, this causes a lot of ambiguous cases that are better
avoided.
One way we could probably improve the error message is to change
it to "template struct test.A(int var = 3) is used as a type. It
must be instantiated", or something along those lines, to make it
clear why you can't use A as a type.
