fix(type-inference)!: combine Expand switching field nullability

[alexandrefimov]

Infer an Expand switching field as nullable when any duplicate is nullable, as required by spec v0.101.0. Taking only the first duplicate made the output schema depend on their order.

Closes #269

BREAKING CHANGE: Expand and unpivot switching fields now infer a nullable output when any duplicate is nullable. Consumers that assumed a required output from the first duplicate must use the combined nullability.

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• src/substrait/type_inference.py
• tests/builders/plan/test_expand.py
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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Corrected type inference for lambda expressions, including nested parameter references and references to input fields.
  • Lambda invocations now infer their result type from the lambda body, and parameter scopes remain isolated from unrelated expressions.
  • Improved inferred nullability for Expand switching fields when multiple alternatives share an expression, including cases where only some alternatives are nullable.
  • Invalid references to lambda scopes now produce an error. Unbound input types remain unbound during Expand inference.

Walkthrough

Type inference tracks lambda parameter scopes and resolves nested parameter references. Expand switching-field inference retains the first duplicate’s type and combines nullability across bound duplicates. Tests cover scope selection, captures, nullability, errors, and non-mutation.

Changes

Type Inference

Layer / File(s)	Summary
Lambda parameter scope resolution src/substrait/type_inference.py, tests/test_type_inference.py	Lambda bodies are inferred with a context-local parameter stack. Parameter references select an enclosing scope using steps_out; references outside the active scopes raise an error. Tests cover nested scopes, input-row references, captures, and scope isolation.
Expand switching-field nullability src/substrait/type_inference.py, tests/builders/plan/test_expand.py, tests/test_type_inference.py	The first duplicate supplies the switching field’s type. If any bound duplicate is nullable, inference marks the output nullable. Tests cover duplicate combinations, unbound types, lambda alternatives, and plan or relation non-mutation.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 902eb

Lambda references resolve to the expected scope, and Expand output is nullable when either duplicate is nullable. No merge-blocking issue was identified.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 168fb

The changes affect inferred query schemas and lambda scope handling. The inspected code restores scopes after success or failure and avoids modifying input types when combining nullability. No new privilege or data-access path was established, but downstream security-sensitive use remains only partially covered.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated propagation is through in-process expression, relation, and plan schema inference and existing builder consumers. Incorrect inferred types could affect downstream schema consumers, but the inspected evidence does not establish a privileged sink or determine maximum tenant, service, or data-store exposure.

Trust Boundaries and Controls

• observed — The new scope mechanism separates lambda parameter identity from captured input-row identity and bounds enclosing-scope selection while a lambda is active. Outside an active lambda, the compatibility fallback uses parent_schema without checking steps_out, so that fallback must not be described as strict scope validation.

Resilience and Maintainability Implications

• observed — Each lambda-body transition owns a ContextVar token and restores the prior scope in finally before returning a function or invocation type. Nested tests cover current and enclosing scopes, and recovery tests assert independent inference after both success and body failure. Context-local isolation is supported by the implementation; dedicated concurrency testing was not established.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 15.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #269 requires an Expand switching field to be nullable when any duplicate is nullable, regardless of duplicate order. infer_rel_schema infers every duplicate, keeps the first duplicate’s type,…
Out of Scope Changes check	✅ Passed	The lambda scope and invocation inference changes support deriving duplicate expression types for Expand switching fields, including lambda bodies and captured input columns. The related tests verify …
Title check	✅ Passed	The title is concise, follows the required Conventional Commit format, and identifies the breaking change to Expand switching-field nullability.
Description check	✅ Passed	The description explains the rationale, references the linked issue, and includes a BREAKING CHANGE footer.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/substrait/type_inference.py:
- Around line 624-627: Update the parent_schema fallback in type inference so it
applies only when the reference has steps_out=0. Check the offset before reading
parent_schema.types, and report a missing enclosing lambda for steps_out=1 when
_lambda_schemas is empty.

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[nielspardon]

Please split the lambda changes (resolving root vs. parameter references inside a lambda body, and the new lambda_invocation support) into their own PR, and keep this one to #269. Widening the switching field changes the inferred schema of existing expand/unpivot plans, so retitle this fix!: … and end the body with a BREAKING CHANGE: footer, as #272 and #279 did. The lambda fixes match the spec, but they change behavior well beyond Expand and add new error paths, so they deserve their own changelog entry.

[nielspardon]

Thanks for splitting this out. Two small follow-ups inline.