Preserve output names for returned input aliases

[TANGBUDU]

Fixes #2714.

When a graph input is returned through a local alias, use the alias name for the inserted Identity output instead of the generic return_val name. Directly returning an unchanged graph input keeps the existing naming behavior. Generated names still go through the existing uniqueness allocator.

The method audit found a missed case: Y = X; X = op.Neg(X); return Y returned the original input under the name X, not Y. Looking up the resolved ONNX name in the mutable Python symbol table incorrectly classified the value after X was rebound. The converter now checks the resolved IR value directly.

Regression coverage includes input-name rebinding, collisions with an existing input name, duplicate alias returns, ONNX checker validation, and runtime output parity. The rebinding regression fails before the follow-up fix ('X' != 'Y') and passes afterward.

Tests (2026-09-07):

• python -m pytest onnxscript/_internal/converter_test.py -q: 56 passed, 1 skipped, 1 xfailed, 3 xpassed; 160 subtests passed, both with ONNX 1.22.0 / ONNX Runtime 1.29.0 and with ONNX 1.18.0 / ONNX Runtime 1.23.0 / ONNX IR 0.1.16.
• lintrunner -a: no lint issues.
• git diff --check: clean.

Revalidation on 2026-09-08 at unchanged head cbcda16aa93281d0c5e5dcb50baa96951e3a2f08: both converter-suite results above were reproduced.

AI assistance was used for the source audit, patch preparation, and validation. The existing expected-failure / unexpected-pass results are reported above, not counted as ordinary passes.

[TANGBUDU]

Justin Chu (@justinchuby) gentle ping on this PR when you have a chance. It addresses #2714 and is ready for review. Happy to address any feedback — thanks!

[codecov]

Codecov Report

❌ Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.70%. Comparing base (3ba2bf7) to head (cbcda16).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/_internal/converter_test.py	80.95%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3018      +/-   ##
==========================================
+ Coverage   72.64%   72.70%   +0.05%     
==========================================
  Files         265      265              
  Lines       32251    32339      +88     
  Branches     3050     3059       +9     
==========================================
+ Hits        23429    23512      +83     
- Misses       7786     7788       +2     
- Partials     1036     1039       +3     

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[Copilot]

🟢 Approval recommended

The change is narrowly scoped, corrects the rebinding misclassification by relying on IR state, and includes comprehensive regression tests for the reported and related edge cases.

Pull request overview

Updates the ONNXScript converter to preserve Python alias names when returning a graph input through a local alias, by using the alias name for the inserted Identity output (while keeping the existing return_val... behavior for direct return X of an input). This addresses the rebinding regression described in #2714 by checking the resolved IR value (ir.Value.is_graph_input()) instead of consulting the mutable Python symbol table.

Changes:

• In Converter._translate_return_stmt, detect returned graph inputs via return_var.is_graph_input() and name the inserted Identity using the return expression’s ast.Name.id when it differs from the resolved ONNX value name.
• Add regression tests covering alias-preserved naming, input rebinding, name collisions with an existing input, and duplicate alias returns (with ONNX checker + runtime parity where relevant).

File summaries

File	Description
onnxscript/_internal/converter.py	Fixes output naming for returned input aliases by deriving the copy name from the returned ast.Name when appropriate and using IR-based graph-input detection.
onnxscript/_internal/converter_test.py	Adds targeted regression tests for alias naming preservation, rebinding correctness, collision handling, and duplicate alias return behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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