# Elixir Digest — 2026-04-30

## Type System / Gradual Typing

### #15324 — Fix invalid type inference on size compare in guards
- **Author:** lukaszsamson
- **Merged:** 2026-04-29
- Mirror expressions like `tuple_size(x) >= 2` and `2 <= tuple_size(x)` were producing different type inferences.
- Fix introduces `mirror_order/1` helper that correctly flips comparison operators when operands are swapped.
- Impact: Guards with size on the right now correctly infer the same type as left-sided equivalents.

### #15322 — Fix map union optimization for open maps
- **Author:** gldubc (Guillaume Duboc)
- **Merged:** 2026-04-29
- Optimizer subtype shortcut incorrectly concluded open map contained by closed map when field types matched.
- Open maps can have extra keys — fix rejects shortcut on tag (open/closed) mismatch.
- Impact: Fixes false type narrowing with unions of open and closed map types.

### #15319 — Fix map difference union optimization
- **Author:** gldubc (Guillaume Duboc)
- **Merged:** 2026-04-28
- Disabled `:union` case of single-key open-map difference optimization.
- Could build invalid union literals causing double negation to leave phantom empty maps.

## Core / Runtime

### #15306 — Fix reentrancy of Code.eval_*
- **Author:** lukaszsamson
- **Merged:** 2026-04-28
- Nested `Code.eval_string` clobbered outer eval's `dbg_callback` and `?elixir_eval_env` in process dict.
- Fix: save/restore pattern (save before, restore in `after` block).
- Discussion: Jonatan Kłosko caught test was not actually testing nested eval; José simplified implementation.
- Lesson: Process dictionary as implicit state = reentrancy bugs. Deliberate trade-off for version decoupling.

### #15316 — Consistently return path as binary in relative_to_cwd
- **Author:** lukaszsamson
- **Merged:** 2026-04-28
- `Path.relative_to_cwd/1` could return chardata on `:file.get_cwd` failure.
- All code paths now normalize to binary.

## Patterns to Extract

- **Process dict save/restore for reentrancy** (#15306): When using process dict as implicit state, always save/restore in try/after to handle reentrancy. The Elixir team chose this over closures to avoid coupling eval to specific Elixir versions.
- **Set-theoretic type system edge cases** (#15322, #15319): Open vs closed map distinction is subtle in BDD-based type representations. Subtype checks must respect structural tags, not just field types.