Add test for rust-lang#102154

Author: aDotInTheVoid

src/test/rustdoc/issue-102154.rs

@@ -0,0 +1,13 @@
+trait A<Y, N> {
+    type B;
+}
+type MaybeBox<T> = <T as A<T, Box<T>>>::B;
+struct P {
+    t: MaybeBox<P>
+}
+impl<Y, N> A<Y, N> for P {
+    type B = N;
+}
+fn main() {
+    let t: MaybeBox<P>;
+}