Dereference small (non recursive) tag unions before storing them elsewhere

Closes #2290

compiler/gen_llvm/src/llvm/build.rs

@@ -1394,6 +1394,18 @@ fn build_wrapped_tag<'a, 'ctx, 'env>(
             );
 
             field_vals.push(ptr);
+        } else if matches!(
+            tag_field_layout,
+            Layout::Union(UnionLayout::NonRecursive(_))
+        ) {
+            debug_assert!(val.is_pointer_value());
+
+            // We store non-recursive unions without any indirection.
+            let reified = env
+                .builder
+                .build_load(val.into_pointer_value(), "load_non_recursive");
+
+            field_vals.push(reified);
         } else {
             // this check fails for recursive tag unions, but can be helpful while debugging
             // debug_assert_eq!(tag_field_layout, val_layout);

compiler/test_gen/src/gen_tags.rs

@@ -1243,3 +1243,22 @@ fn tag_must_be_its_own_type() {
         i64
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm"))]
+fn recursive_tag_union_into_flat_tag_union() {
+    // Comprehensive test for correctness in cli/tests/repl_eval
+    assert_evals_to!(
+        indoc!(
+            r#"
+            Item : [ Shallow [ L Str, R Str ], Deep Item ]
+            i : Item
+            i = Deep (Shallow (R "woo"))
+            i
+            "#
+        ),
+        0,
+        usize,
+        |_| 0
+    )
+}