test: Adda test that all arms of an alt may result in fail

src/test/run-pass/expr-alt-fail-all.rs

@@ -0,0 +1,20 @@
+// When all branches of an alt expression result in fail, the entire
+// alt expression results in fail.
+
+fn main() {
+  auto x = alt (true) {
+    case (true) {
+      10
+    }
+    case (true) {
+      alt (true) {
+        case (true) {
+          fail
+        }
+        case (false) {
+          fail
+        }
+      }
+    }
+  };
+}