#1038 S4: Lowering/runtime route for return-side CASE execution

[lmeyerov]

Summary

Execution slice S4 for meta #1261 and source residual #1038.

Implement/validate lowering + runtime route for admitted return-side CASE shapes end-to-end.

Parent/Related

• Meta: #1261
• Source residual: #1038

Scope

• Lowering path for CASE in RETURN items
• Runtime row-expression evaluation compatibility
• Preserve behavior for existing non-CASE and already-supported CASE paths

Acceptance Criteria

• Admitted CASE-return shape executes end-to-end in local Cypher path
• Regression tests cover positive/negative paths
• No regressions in nearby IC4 CASE lanes

[lmeyerov]

S4 lowering/runtime progress update (branch issue-1261-case-return-decomp):

Identified residual behavior gap: aggregate calls nested in row CASE expressions (e.g. CASE ... THEN count(*) ELSE ... END) leaked through lowering and failed at runtime select evaluation.

Implemented fix:

• graphistry/compute/gfql/cypher/lowering.py
  • _row_expr_arg(...) now fails fast when aggregate calls appear in row expressions, with compile-time unsupported diagnostics (E108) instead of runtime type errors.

Added regression lock:

• graphistry/tests/compute/gfql/cypher/test_lowering.py
  • test_issue_1038_rejects_aggregate_inside_row_case_expression

Validation evidence:

• PYTHONPATH=. python -m pytest graphistry/tests/compute/gfql/cypher/test_lowering.py -k "issue_1038_rejects_aggregate_inside_row_case_expression or issue_1038_ic4_return_side_case_expression_regression_lock" -q → 2 passed

Commit:

• 626bf418 (fix(cypher): fail fast on aggregates nested in row CASE expressions)

[lmeyerov]

Closed by merged PR #1271 (commit 74efb35): lowering/runtime guarded route delivered in-scope.