Add test for issue #78671

src/test/ui/generic-associated-types/issue-78671.rs

@@ -0,0 +1,14 @@
+#![allow(incomplete_features)]
+#![feature(generic_associated_types)]
+
+trait CollectionFamily {
+    type Member<T>;
+         //~^ ERROR: missing generics for associated type
+}
+fn floatify() {
+    Box::new(Family) as &dyn CollectionFamily<Member=usize>
+}
+
+struct Family;
+
+fn main() {}

src/test/ui/generic-associated-types/issue-78671.stderr

@@ -0,0 +1,19 @@
+error[E0107]: missing generics for associated type `CollectionFamily::Member`
+  --> $DIR/issue-78671.rs:5:10
+   |
+LL |     type Member<T>;
+   |          ^^^^^^ expected 1 type argument
+   |
+note: associated type defined here, with 1 type parameter: `T`
+  --> $DIR/issue-78671.rs:5:10
+   |
+LL |     type Member<T>;
+   |          ^^^^^^ -
+help: use angle brackets to add missing type argument
+   |
+LL |     type Member<T><T>;
+   |                ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0107`.