Closed
Description
spawned off of #21259
test case
use std::default::Default;
trait Impled { fn f(&self) {} }
trait Vr: Impled {
type Y: Default + Par<<Self as Impled>::Noexist>; fn g(&self) {}
}
trait Par<X:Default> {
fn h(&self) -> (X, String) { (Default::default(), format!("Par::h")) }
}
struct S1;
impl Impled for S1 {}
impl Vr for S1 { type Y = (); }
impl Par<()> for () { }
fn main() {
let y: <S1 as Vr>::Y = Default::default();
let (_, s) = Par::h(&y);
println!("{}", s);
}The above code should be rejected by the compiler, because <Self as Impled>::Noexist is not a well-formed projection of an associated item. Instead, it currently runs.
(This bug is not quite as bad as it might seem, since one easily runs into problems if one actually attempts to program generically with this trait and associated item, as shown in the below modification to the last three lines above. This new program is correctly rejected by the compiler, though perhaps with messages that are ... subpar:)
fn foo<V:Vr>(v: V) {
let y: <V as Vr>::Y = Default::default();
let (_, s) = Par::h(&y);
println!("{}", s);
}
fn main() {
foo(S1);
}Transcript of error output:
../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 error: the trait `core::marker::Sized` is not implemented for the type `<V as Impled>::Noexist` [E0277]
../src/test/compile-fail/issue-21259-e.rs:18 let (_, s) = Par::h(&y);
^~~~~~
../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 note: `<V as Impled>::Noexist` does not have a constant size known at compile-time
../src/test/compile-fail/issue-21259-e.rs:18 let (_, s) = Par::h(&y);
^~~~~~
../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 error: the trait `core::default::Default` is not implemented for the type `<V as Impled>::Noexist` [E0277]
../src/test/compile-fail/issue-21259-e.rs:18 let (_, s) = Par::h(&y);
^~~~~~
error: aborting due to 2 previous errors