Add a regression test for issue-81712

src/test/ui/generic-associated-types/issue-81712-cyclic-traits.rs

@@ -0,0 +1,21 @@
+// Regression test for #81712.
+
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+trait A {
+    type BType: B<AType = Self>;
+}
+
+trait B {
+    type AType: A<BType = Self>;
+}
+trait C {
+    type DType<T>: D<T, CType = Self>;
+    //~^ ERROR: missing generics for associated type `C::DType` [E0107]
+}
+trait D<T> {
+    type CType: C<DType = Self>;
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-81712-cyclic-traits.stderr

@@ -0,0 +1,19 @@
+error[E0107]: missing generics for associated type `C::DType`
+  --> $DIR/issue-81712-cyclic-traits.rs:14:10
+   |
+LL |     type DType<T>: D<T, CType = Self>;
+   |          ^^^^^ expected 1 type argument
+   |
+note: associated type defined here, with 1 type parameter: `T`
+  --> $DIR/issue-81712-cyclic-traits.rs:14:10
+   |
+LL |     type DType<T>: D<T, CType = Self>;
+   |          ^^^^^ -
+help: use angle brackets to add missing type argument
+   |
+LL |     type DType<T><T>: D<T, CType = Self>;
+   |               ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0107`.