"list index out of range" failure instead of Unrecognized term of (...) error msg

[juankx-bodo]

PyDough fails with an unhandled IndexError: list index out of range when a partitioned variable name (c1) is referenced inside its own CALCULATE expression, rather than raising a user-friendly domain error message (e.g., Unrecognized term...).

Traceback (most recent call last):
  File "/home/juankx/svc/PyDough/pydough_test.py", line 182, in <module>
    sql = pydough.to_sql(unqualified_node, session=tester.session)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/evaluation/evaluate_unqualified.py", line 166, in to_sql
    relational: RelationalRoot = convert_ast_to_relational(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1888, in convert_ast_to_relational
    hybrid: HybridTree = hybrid_translator.convert_qdag_to_hybrid(node)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 1751, in convert_qdag_to_hybrid
    hybrid: HybridTree = self.make_hybrid_tree(node, None)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 1469, in make_hybrid_tree
    hybrid = self.make_hybrid_tree(
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 1472, in make_hybrid_tree
    self.populate_children(hybrid, node, child_ref_mapping)
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 422, in populate_children
    subtree: HybridTree = self.make_hybrid_tree(
                          ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 1640, in make_hybrid_tree
    self.populate_children(
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 422, in populate_children
    subtree: HybridTree = self.make_hybrid_tree(
                          ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/hybrid_translator.py", line 1634, in make_hybrid_tree
    HybridPartitionChild(source.children[0].subtree),
                         ~~~~~~~~~~~~~~~^^^
IndexError: list index out of range

Minimal reproducible example:

# Raises "IndexError: list index out of range"
c1 = customers.PARTITION(name="by_marketsegment", by=market_segment)
r1 = c1.CALCULATE(x=COUNT(c1))

LLM Explanation Discrepancy

While code execution fails with an IndexError, calling pydough.explain_llm() on the exact same code completes without reporting an error ("error": false):

{
  "error": false,
  "query_summary": "Accesses 'customers', partitioned by market_segment, computing counting customers records per group as 'x'."
}

Expected Behavior:

If c1 is out-of-scope or invalid within the expression c1.CALCULATE(...), PyDough should raise a descriptive context error instead of crashing with a generic IndexError:

Unrecognized term of TPCH.Partition(customers, name='by_marketsegment', by=market_segment): 'c1'. Did you mean: customers, market_segment?

Additional Context / Comparison:

Standard error reporting works as expected when referencing undefined or out-of-scope terms in other partition scenarios (r1 and r2):

c1 = customers.PARTITION(name="by_marketsegment", by=market_segment)
c2 = suppliers.PARTITION(name="by_nationname", by=nation.name)
r1 = c1.CALCULATE(x=COUNT(c3))
r2 = c2.CALCULATE(x=COUNT(c1))

Schema Context (TPCH Collections Used):

 customers
  └── market_segment

  nations
  ├── name
  └── suppliers [multiple customers]

  suppliers
  └── nation [one member of nations]