Add test for issue rust-lang#71176

Author: marmeladema

src/test/ui/generic-associated-types/issue-71176.rs

@@ -0,0 +1,21 @@
+#![allow(incomplete_features)]
+#![feature(generic_associated_types)]
+
+trait Provider {
+    type A<'a>;
+      //~^ ERROR: missing generics for associated type
+}
+
+impl Provider for () {
+    type A<'a> = ();
+}
+
+struct Holder<B> {
+  inner: Box<dyn Provider<A = B>>,
+}
+
+fn main() {
+    Holder {
+        inner: Box::new(()),
+    };
+}

src/test/ui/generic-associated-types/issue-71176.stderr

@@ -0,0 +1,19 @@
+error[E0107]: missing generics for associated type `Provider::A`
+  --> $DIR/issue-71176.rs:5:10
+   |
+LL |     type A<'a>;
+   |          ^ expected 1 lifetime argument
+   |
+note: associated type defined here, with 1 lifetime parameter: `'a`
+  --> $DIR/issue-71176.rs:5:10
+   |
+LL |     type A<'a>;
+   |          ^ --
+help: use angle brackets to add missing lifetime argument
+   |
+LL |     type A<'a><'a>;
+   |           ^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0107`.