Add test.

tests/ui/self/self-ctor-nongeneric.rs

@@ -0,0 +1,15 @@
+// `Self` as a constructor is currently allowed when the outer item is not generic.
+// check-pass
+
+struct S0(usize);
+
+impl S0 {
+    fn foo() {
+        const C: S0 = Self(0);
+        fn bar() -> S0 {
+            Self(0)
+        }
+    }
+}
+
+fn main() {}