fix(optimizer): prevent Shape/Gather from folding to constants when shape has negative (unknown) dims

[Copilot]

FoldConstantsPass incorrectly folds Shape→Gather→Concat→Reshape patterns when the input tensor has unknown dimensions stored as integer -1 in the IR (as opposed to ir.SymbolicDim). Since -1 is an int, the all-integer check passes, causing Shape to fold into Constant(value_ints=[-1, 512, 1, ...]). Gather then returns a constant -1, which propagates into the Reshape shape — producing an invalid shape like [-1, -1] (ONNX allows at most one -1 in a Reshape shape).

Changes

• Shape handler: add non-negativity guard — only fold to a constant when all shape dims are known positive integers:

  # before
  if all(isinstance(d, int) for d in shape_slice):
  # after
  if all(isinstance(d, int) and d >= 0 for d in shape_slice):

• Gather handler: same guard on the symbolic-shape gather path, preventing folding when any gathered dimension value is negative.
• Regression test: test_shape_gather_concat_reshape_with_neg1_dim — verifies that Shape is preserved and Reshape never receives multiple -1 values when the input shape contains -1 integer dims.

The root cause: ONNX IR deserializes unknown dims (dim_value=-1 in the proto) as integer -1, not ir.SymbolicDim. The existing SymbolicDim path was safe; the -1-as-int path was not.

[Copilot]

Pull request overview

This PR hardens the optimizer’s FoldConstantsPass against ONNX-IR shapes that encode unknown dimensions as the integer -1, preventing invalid constant-folding that can propagate multiple -1 values into Reshape shapes.

Changes:

• Add a non-negativity guard to Shape constant folding so -1 (unknown) dims do not become literal constants.
• Add the same non-negativity guard to the Gather-from-symbolic-shape folding path to avoid folding negative gathered dims.
• Add a regression test covering the Shape→Gather→Concat→Reshape pattern when the input shape contains integer -1 dims.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
onnxscript/optimizer/_constant_folding.py	Prevents folding Shape/Gather results to constants when any folded dimension is negative (unknown).
onnxscript/optimizer/_constant_folding_test.py	Adds regression coverage for the -1-dim folding bug scenario involving Shape/Gather/Reshape.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.61%. Comparing base (d3489d4) to head (61b855d).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/optimizer/_constant_folding_test.py	71.42%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2964      +/-   ##
==========================================
- Coverage   72.61%   72.61%   -0.01%     
==========================================
  Files         263      263              
  Lines       32034    32048      +14     
  Branches     3013     3014       +1     
==========================================
+ Hits        23263    23273      +10     
- Misses       7748     7751       +3     
- Partials     1023     1024       +1     

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