fix: preserve boolean checks through TCO#817
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stephenamar-db merged 1 commit intodatabricks:masterfrom May 1, 2026
Merged
fix: preserve boolean checks through TCO#817stephenamar-db merged 1 commit intodatabricks:masterfrom
stephenamar-db merged 1 commit intodatabricks:masterfrom
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Motivation: The straightforward fix would be to resolve a TailCall inside visitAndRhsTailPos/visitOrRhsTailPos and then call asBoolean. That path was intentionally avoided: nested resolution re-enters the trampoline from inside the current function body, can restore stack growth for deep &&/|| chains, and can move the lazy evaluation boundary. We still need recursive TailCall rhs values to obey normal boolean operator result checks. Modification: Carry &&/|| result validation as delayed TailCall state and apply it only when the outer TailCall.resolve loop produces the final value. Keep auto-TCO arguments under TailstrictModeAutoTCO and add non-boolean TailCall rhs regression tests for both && and ||. Result: Deep boolean auto-TCO chains remain stack-safe and lazy, while recursive rhs values that resolve to non-booleans now report the same type errors as direct true && 0 / false || 0 expressions. Risk: The main risk is accidentally replacing this with immediate nested resolution later. Code comments now document why the delayed check lives on TailCall instead of resolving inside the &&/|| helpers. Error-value tail propagation is intentionally left as-is in this PR; the existing directional fixture is restored but remains a non-asserted coverage gap for a follow-up.
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Motivation
Recursive calls in tail-position
&&/||rhs can return through the TailCall trampoline. Before this change, direct non-boolean rhs values were rejected, but non-boolean values produced after resolving a TailCall could skip the normal boolean operator result check.The obvious fix would be to call
TailCall.resolve(tc).asBooleaninsidevisitAndRhsTailPos/visitOrRhsTailPos. That path is intentionally avoided: nested resolution re-enters the trampoline from inside the current function body, can restore stack growth for deep&&/||chains, and can move the lazy evaluation boundary.Modification
&&/||result validation as delayedTailCallstate and apply it only when the outerTailCall.resolveloop produces the final value.TailstrictModeAutoTCO; the helper only attaches a delayed check and does not force nested TailCalls.&&and||.Result
errorarguments through both&&and||still stay unevaluated.true && 0/false || 0expressions.Verification:
./mill -i __.test(SUCCESS, 176s)./mill -i __.checkFormat(SUCCESS, 6s)git diff --checkRisk
The main risk is that a future cleanup replaces the delayed check with immediate nested TailCall resolution. This PR documents the reason in code comments near both the
&&/||tail helpers andTailCall.BooleanCheck.Error-value tail propagation is intentionally left as-is in this PR. The existing
auto_tco_directional.jsonnetfixture entry is restored, but it is not asserted today; real coverage for that path should be handled in a follow-up so this PR stays focused on boolean operator correctness.