Handle case where only other tag of nullable unwrapped needs no refcount

It's possible to have a nullable unwrapped code where the only material
payload does not need refcounting operations. In this case the
refcounting function is the trivial one.

crates/compiler/gen_llvm/src/llvm/refcounting.rs

@@ -1345,11 +1345,16 @@ fn build_rec_union_recursive_decrement<'a, 'ctx, 'env>(
         union_layout,
         UnionLayout::NullableUnwrapped { .. } | UnionLayout::NonNullableUnwrapped { .. }
     ) {
-        debug_assert_eq!(cases.len(), 1);
+        debug_assert!(cases.len() <= 1, "{cases:?}");
 
-        // in this case, don't switch, because the `else` branch below would try to read the (nonexistent) tag id
-        let (_, only_branch) = cases.pop().unwrap();
-        env.builder.build_unconditional_branch(only_branch);
+        if cases.is_empty() {
+            // The only other layout doesn't need refcounting. Pass through.
+            builder.build_return(None);
+        } else {
+            // in this case, don't switch, because the `else` branch below would try to read the (nonexistent) tag id
+            let (_, only_branch) = cases.pop().unwrap();
+            env.builder.build_unconditional_branch(only_branch);
+        }
     } else {
         let default_block = env.context.append_basic_block(parent, "switch_default");
 
@@ -1372,6 +1377,7 @@ fn build_rec_union_recursive_decrement<'a, 'ctx, 'env>(
     }
 }
 
+#[derive(Debug)]
 struct UnionLayoutTags<'a> {
     nullable_id: Option<u16>,
     tags: &'a [&'a [InLayout<'a>]],

crates/compiler/test_gen/src/gen_tags.rs

@@ -2196,3 +2196,42 @@ fn nullable_wrapped_with_nullable_not_last_index() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn refcount_nullable_unwrapped_needing_no_refcount_issue_5027() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            Effect : {} -> Str
+
+            after = \effect, buildNext ->
+                \{} ->
+                    when buildNext (effect {}) is
+                        thunk -> thunk {}
+
+            line : Effect
+            line = \{} -> "done"
+
+            await : Effect, (Str -> Effect) -> Effect
+            await = \fx, cont ->
+                after
+                fx
+                cont
+
+            succeed : {} -> Effect
+            succeed = \{} -> (\{} -> "success")
+
+            test =
+                await line \s ->
+                    if s == "done" then succeed {} else test
+
+            main = test {}
+            "#
+        ),
+        RocStr::from("success"),
+        RocStr
+    );
+}