add test for trait alias projections

crates/hir-ty/src/tests/trait_aliases.rs

@@ -0,0 +1,21 @@
+use crate::tests::check_types;
+
+#[test]
+fn projection() {
+    check_types(
+        r#"
+#![feature(trait_alias)]
+
+pub trait A {
+    type Output;
+}
+
+pub trait B = A<Output = u32>;
+
+pub fn a<T: B>(x: T::Output) {
+    x;
+//  ^ u32
+}
+"#,
+    );
+}