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//! Tests the interaction of associated type defaults and specialization.
#![feature(associated_type_defaults, specialization)]
//~^ WARN the feature `specialization` is incomplete
trait Tr {
type Ty = u8;
fn make() -> Self::Ty {
0u8
//~^ error: mismatched types
}
}
struct A<T>(T);
// In a `default impl`, assoc. types are defaulted as well,
// so their values can't be assumed.
default impl<T> Tr for A<T> {
fn make() -> u8 { 0 }
//~^ ERROR method `make` has an incompatible type for trait
}
struct A2<T>(T);
// ...same, but in the method body
default impl<T> Tr for A2<T> {
fn make() -> Self::Ty { 0u8 }
//~^ ERROR mismatched types
}
struct B<T>(T);
// Explicitly defaulting the type does the same.
impl<T> Tr for B<T> {
default type Ty = bool;
fn make() -> bool { true }
//~^ ERROR method `make` has an incompatible type for trait
}
struct B2<T>(T);
// ...same, but in the method body
impl<T> Tr for B2<T> {
default type Ty = bool;
fn make() -> Self::Ty { true }
//~^ ERROR mismatched types
}
struct C<T>(T);
// Only the method is defaulted, so this is fine.
impl<T> Tr for C<T> {
type Ty = bool;
default fn make() -> bool { true }
}
// Defaulted method *can* assume the type, if the default is kept.
struct D<T>(T);
impl<T> Tr for D<T> {
default fn make() -> u8 { 0 }
}
impl Tr for D<bool> {
fn make() -> u8 { 255 }
}
struct E<T>(T);
impl<T> Tr for E<T> {
default type Ty = bool;
default fn make() -> Self::Ty { panic!(); }
}
// This impl specializes and sets `Ty`, it can rely on `Ty=String`.
impl Tr for E<bool> {
type Ty = String;
fn make() -> String { String::new() }
}
fn main() {
// Test that we can assume the right set of assoc. types from outside the impl
// This is a `default impl`, which does *not* mean that `A`/`A2` actually implement the trait.
// cf. https://github.com/rust-lang/rust/issues/48515
//let _: <A<()> as Tr>::Ty = 0u8;
//let _: <A2<()> as Tr>::Ty = 0u8;
let _: <B<()> as Tr>::Ty = 0u8; //~ error: mismatched types
let _: <B<()> as Tr>::Ty = true; //~ error: mismatched types
let _: <B2<()> as Tr>::Ty = 0u8; //~ error: mismatched types
let _: <B2<()> as Tr>::Ty = true; //~ error: mismatched types
let _: <C<()> as Tr>::Ty = true;
let _: <D<()> as Tr>::Ty = 0u8;
let _: <D<bool> as Tr>::Ty = 0u8;
}
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