Add ui test for projection used as union field type

tests/ui/union/projection-as-union-type.rs

@@ -0,0 +1,19 @@
+// Ensures that we can use projections as union field's type.
+// check-pass
+
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity;
+}
+
+impl<T> Identity for T {
+    type Identity = Self;
+}
+
+pub type Foo = u8;
+
+pub union Bar {
+    pub a: <Foo as Identity>::Identity,
+    pub b: u8,
+}