changelog: 2026-04-30 digest (initial)

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+# 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.