test: Add a test case for derived-tydescs-before-dynamic-allocas

src/test/run-pass/alloca-from-derived-tydesc.rs

@@ -0,0 +1,17 @@
+// xfail-stage0
+
+tag option[T] {
+    some(T);
+    none;
+}
+
+type r[T] = rec(mutable (option[T])[] v);
+
+fn f[T]() -> T[] {
+    ret ~[];
+}
+
+fn main() {
+    let r[int] r = rec(mutable v=~[]);
+    r.v = f();
+}