Fix max_sz bug that ended up causing us to index incorrectly into a vec of tag types. Add a testcase.

src/Makefile

@@ -507,6 +507,7 @@ TEST_XFAILS_LLVM := $(TASK_XFAILS) \
                       rec-tup.rs \
                       rec.rs \
                       simple-obj.rs \
+                      size-and-align.rs \
                       spawn-fn.rs \
                       spawn-module-qualified.rs \
                       spawn.rs \

src/boot/util/common.ml

@@ -686,7 +686,7 @@ let rec max_sz (a:size) (b:size) : size =
       | (SIZE_rt_max (b, c), a) when a = c -> max_sz a b
       | (SIZE_fixed a, SIZE_fixed b) -> SIZE_fixed (i64_max a b)
       | (SIZE_fixed 0L, b) when no_negs b -> b
-      | (a, SIZE_fixed 0L) when no_negs a -> b
+      | (a, SIZE_fixed 0L) when no_negs a -> a
       | (a, SIZE_fixed b) -> max_sz (SIZE_fixed b) a
       | (a, b) when a = b -> a
       | (a, b) -> SIZE_rt_max (a, b)

src/test/run-pass/size-and-align.rs

@@ -0,0 +1,18 @@
+// -*- rust -*-
+
+use std;
+import std._vec;
+
+type clam[T] = tag(a(T, int), b());
+
+fn uhoh[T](vec[clam[T]] v) {
+  alt (v.(1)) {
+    case (a[T](t, u)) { log "incorrect"; log u; fail; }
+    case (b[T]()) { log "correct"; }
+  }
+}
+
+fn main() {
+  let vec[clam[int]] v = vec(b[int](), b[int](), a[int](42, 17));
+  uhoh[int](v);
+}