Added test for issue rust-lang#56488.

Author: alexreg

src/test/ui/issues/issue-56488.rs

@@ -0,0 +1,13 @@
+// run-pass
+
+#![feature(trait_alias)]
+
+mod alpha {
+    pub trait A {}
+    pub trait C = A;
+}
+
+#[allow(unused_imports)]
+use alpha::C;
+
+fn main() {}