Add test for issue-48638

Author: JohnTitor

src/test/ui/wf/issue-48638.rs

@@ -0,0 +1,21 @@
+// check-pass
+
+pub trait D {}
+pub struct DT;
+impl D for DT {}
+
+pub trait A<R: D>: Sized {
+    type AS;
+}
+
+pub struct As<R: D>(R);
+
+pub struct AT;
+impl<R: D> A<R> for AT {
+    type AS = As<R>;
+}
+
+#[repr(packed)]
+struct S(<AT as A<DT>>::AS);
+
+fn main() {}