Closed
Description
The following should compile correctly AFAIK, but the type parameter isn't being properly inferred.
Updated Testcase
use std::io::println;
mod base {
pub trait HasNew<T> {
#[cfg(buggy)]
fn new() -> T;
#[cfg(fixed)]
fn new() -> Self; // (see niko's first comment in ticket)
}
#[deriving(Show)]
pub struct Foo {
dummy: (),
}
impl HasNew<Foo> for Foo {
fn new() -> Foo {
Foo { dummy: () }
}
}
pub struct Bar {
dummy: (),
}
impl HasNew<Bar> for Bar {
fn new() -> Bar {
Bar { dummy: () }
}
}
}
fn main() {
// 1. You cannot say this:
// let f: base::Foo = base::HasNew<base::Foo>::new();
// 2. There are discussions to provide some way to say
// something like this (see Issue 6894):
// let f: base::Foo = base::HasNew<base::Foo, for base::Foo>::new();
// 3. But for now, one must rely on the type inference system
// to feed the appropriate type via the expression context in which
// the invocation of `new` occurs:
let f: base::Foo = base::HasNew::new();
println(format!("{}", f).as_slice());
}Original Outdated Testcase
mod base {
trait HasNew<T> {
static pure fn new() -> T;
}
pub struct Foo {
dummy: (),
}
pub impl Foo : HasNew<Foo> {
static pure fn new() -> Foo {
Foo { dummy: () }
}
}
pub struct Bar {
dummy: (),
}
pub impl Bar : HasNew<Bar> {
static pure fn new() -> Bar {
Bar { dummy: () }
}
}
}
fn main() {
let f: base::Foo = base::new::<base::Foo, base::Foo>();
debug!("%?", f);
}
Error
% rustc --cfg buggy /tmp/base.rs
/tmp/base.rs:42:23: 42:40 error: cannot determine a type for this bounded type parameter: unconstrained type
/tmp/base.rs:42 let f: base::Foo = base::HasNew::new();
^~~~~~~~~~~~~~~~~