fix(compiler2): widen empty-collection branches in control-flow joins (B-236)

[codeshaunted]

Follow-up to #3779. That PR fixed several adjacent soundness holes found while investigating B-236, but the actual reported repro still crashed — this PR fixes the real root cause.

The bug

function f(xs: string[], m: int) -> string {
  let top = if (m > 0) { xs.slice(0, m) } else { [] };
  top.join(" ")   // VM internal error: expected map, got array
}

The if/else joined string[] (then) with the empty-array never[] (else) to the union string[] | never[]. A list-union lowers to a non-List runtime type, so method dispatch resolves array methods (.join) to the map implementation — and the VM aborts when handed an array.

The fix

join_types now recurses into matching container constructors instead of building a union:

• List + List → List<join(elem)>
• Map + Map → Map<join(key), join(value)>

An empty [] / {} has never element/key/value types, which join away, so if c { xs } else { [] } widens to string[] (the issue's own suggested fix). Two list branches of differing element types likewise widen to (a | b)[] rather than a[] | b[].

Notes

• The std library itself hit this pattern (OrchestrationStep[] | never[] → OrchestrationStep[]) — a latent bug this also fixes. TIR + corpus-bytecode snapshots updated; both diffs are exactly T[] | never[] → T[].
• Regression test added (test-first): the repro now returns "a b".
• Full workspace suite green (5891), baml_src corpus green, fmt + clippy clean.

Summary by CodeRabbit

• Bug Fixes
  • Fixed type handling for conditional expressions that combine lists with empty else branches, preventing runtime errors when using containers in control flow operations.

[linear]

B-236

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

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Run ID: d8e44f76-3221-4c8c-8cee-733cf6eee6e5

📥 Commits

Reviewing files that changed from the base of the PR and between 8921e4c and fac79f4.

⛔ Files ignored due to path filters (2)

• baml_language/crates/baml_tests/snapshots/baml_src/patterns_new_runtime.snap is excluded by !**/*.snap
• baml_language/crates/baml_tests/snapshots/compiles/__baml_std__/baml_tests__compiles____baml_std____04_tir.snap is excluded by !**/*.snap

📒 Files selected for processing (2)

• baml_language/crates/baml_compiler2_tir/src/builder.rs
• baml_language/crates/baml_tests/tests/type_error_repro.rs

---

📝 Walkthrough

Walkthrough

join_types in the TIR builder gains an early match block that detects same-kind container pairs (List/EvolvingList or Map/EvolvingMap) and returns the container with recursively joined inner types instead of producing a union. A regression test for B-236 is added to verify the fix.

Changes

Container type join fix (B-236)

Layer / File(s)	Summary
join_types container special-case and regression test baml_language/crates/baml_compiler2_tir/src/builder.rs, baml_language/crates/baml_tests/tests/type_error_repro.rs	Adds an early match in join_types that handles matching List/EvolvingList and Map/EvolvingMap pairs by recursively joining their element/key/value types, preventing degradation to Ty::Union. The regression test if_else_concrete_list_with_empty_else_branch_runs exercises an if/else that combines string[] with [] and asserts .join(" ") returns "a b".

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A rabbit hopped left, a rabbit hopped right,
Two lists in a branch—which type is right?
No union of emptiness shall spoil the stew,
We join them together, both through and through!
🐇 string[] | [] → just string[], hooray!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix(compiler2): widen empty-collection branches in control-flow joins (B-236)' accurately describes the main change: fixing a runtime crash by widening empty collection branches in control-flow type joins, with specific reference to the issue number.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch avery/b-236-join-fix

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