Add test for issue #78561

Author: samlich

src/test/ui/nll/issue-78561.rs

@@ -0,0 +1,23 @@
+// check-pass
+#![feature(type_alias_impl_trait)]
+
+pub trait Trait {
+    type A;
+
+    fn f() -> Self::A;
+}
+
+pub trait Tr2<'a, 'b> {}
+
+pub struct A<T>(T);
+pub trait Tr {
+    type B;
+}
+
+impl<'a, 'b, T: Tr<B = dyn Tr2<'a, 'b>>> Trait for A<T> {
+    type A = impl core::fmt::Debug;
+
+    fn f() -> Self::A {}
+}
+
+fn main() {}