Another test for the occurs check, this one from issue 778

Author: catamorphism

src/test/compile-fail/occurs-check-3.rs

@@ -0,0 +1,4 @@
+// error-pattern:mismatched types
+// From Issue #778
+tag clam[T] { a(T); }
+fn main() { let c = a(c); alt c { a[int](_) { } } }