Return pos impl trait

src/test/ui/const-generics/defaults/rp_impl_trait.rs

@@ -0,0 +1,31 @@
+// run-pass
+#![feature(const_generics_defaults)]
+
+struct Uwu<const N: u32 = 1, const M: u32 = N>;
+
+trait Trait {}
+impl<const N: u32> Trait for Uwu<N> {}
+
+fn rawr<const N: u32>() -> impl Trait {
+    Uwu::<N>
+}
+
+trait Traitor<const N: u8 = 1, const M: u8 = N> { }
+
+impl<const N: u8> Traitor<N> for u32 {}
+impl Traitor<1, 1> for u64 {}
+
+fn uwu<const N: u8>() -> impl Traitor<N> {
+    1_u32
+}
+
+fn owo() -> impl Traitor {
+    1_u64
+}
+
+fn main() {
+    rawr::<3>();
+    rawr::<7>();
+    uwu::<{ u8::MAX }>();
+    owo();
+}

src/test/ui/const-generics/defaults/rp_impl_trait_fail.rs

@@ -0,0 +1,33 @@
+#![feature(const_generics_defaults)]
+
+struct Uwu<const N: u32 = 1, const M: u32 = N>;
+
+trait Trait {}
+impl<const N: u32> Trait for Uwu<N> {}
+
+fn rawr() -> impl Trait {
+    //~^ error: the trait bound `Uwu<10_u32, 12_u32>: Trait` is not satisfied
+    Uwu::<10, 12>
+}
+
+trait Traitor<const N: u8 = 1, const M: u8 = N> { }
+
+impl<const N: u8> Traitor<N, 2> for u32 {}
+impl Traitor<1, 2> for u64 {}
+
+
+fn uwu<const N: u8>() -> impl Traitor<N> {
+    //~^ error: the trait bound `u32: Traitor<N, N>` is not satisfied
+    1_u32
+}
+
+fn owo() -> impl Traitor {
+    //~^ error: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
+    1_u64
+}
+
+fn main() {
+    rawr();
+    uwu();
+    owo();
+}

src/test/ui/const-generics/defaults/rp_impl_trait_fail.stderr

@@ -0,0 +1,30 @@
+error[E0277]: the trait bound `Uwu<10_u32, 12_u32>: Trait` is not satisfied
+  --> $DIR/rp_impl_trait_fail.rs:8:14
+   |
+LL | fn rawr() -> impl Trait {
+   |              ^^^^^^^^^^ the trait `Trait` is not implemented for `Uwu<10_u32, 12_u32>`
+   |
+   = help: the following implementations were found:
+             <Uwu<N> as Trait>
+
+error[E0277]: the trait bound `u32: Traitor<N, N>` is not satisfied
+  --> $DIR/rp_impl_trait_fail.rs:19:26
+   |
+LL | fn uwu<const N: u8>() -> impl Traitor<N> {
+   |                          ^^^^^^^^^^^^^^^ the trait `Traitor<N, N>` is not implemented for `u32`
+   |
+   = help: the following implementations were found:
+             <u32 as Traitor<N, 2_u8>>
+
+error[E0277]: the trait bound `u64: Traitor<1_u8, 1_u8>` is not satisfied
+  --> $DIR/rp_impl_trait_fail.rs:24:13
+   |
+LL | fn owo() -> impl Traitor {
+   |             ^^^^^^^^^^^^ the trait `Traitor<1_u8, 1_u8>` is not implemented for `u64`
+   |
+   = help: the following implementations were found:
+             <u64 as Traitor<1_u8, 2_u8>>
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.