fix(sl): parse user filter expressions as predicates, not projections (#307)

* fix(sl): parse user filter expressions as predicates, not projections

User-authored filters and segments were parsed in a projection context
(`SELECT {expr}`). On T-SQL a top-level `col = 'value'` projection is the
`alias = expression` aliasing syntax, so an equality filter parsed this way
became `'value' AS col` — dropping the comparison entirely and silently
skipping computed-column expansion (the column hid behind the alias).

Parse user fragments as predicates (`SELECT * WHERE {expr}`) at every parse
site — the parser cache, measure-filter CASE WHEN generation, computed-column
expansion, and measure-filter/segment column qualification. For plain
non-condition expressions the column set is identical, so this is a no-op
everywhere except the T-SQL alias case it fixes.

Add cross-dialect regression tests (tsql, postgres, snowflake, bigquery)
locking equality filters/segments to comparison shape and confirming `= 'x'`
now matches `IN ('x')` on T-SQL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Shorten T-SQL predicate comments

* docs(sl): tighten T-SQL predicate docstrings and AGENTS docstring rule

Trim the parser and regression-test docstrings to the 1-3 line bar and
extend the AGENTS.md comment guidance to cover docstrings explicitly.

* refactor(sl): route all filter parsing through parse_predicate

Consolidate the predicate-context parse into a single parse_predicate
helper and route every filter-parsing call site through it: measure
CASE-WHEN filters, segments, computed-column-in-filter, the
aggregate-locality HAVING rewrite, and the planner OR-mixing /
top-level-AND split. The locality and split paths still parsed user
filters in projection context, so a named-measure equality filter
compiled to `0 AS measure` on T-SQL. Add a locality regression test
covering the HAVING rewrite path.

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Andrey Avtomonov <andreybavt@gmail.com>

python/ktx-sl/tests/test_tsql_filter_alias_regression.py

@@ -0,0 +1,175 @@
+"""Regression tests for T-SQL `=` filters mis-parsed as column aliases.
+
+A top-level `col = 'value'` is T-SQL's `alias = expression` projection form, so
+filters and segments must compile as predicates, not projections, on any dialect.
+"""
+
+from __future__ import annotations
+
+import pytest
+import sqlglot
+
+from .conftest import make_engine
+
+
+def _jobs_source(**overrides):
+    base = {
+        "name": "jobs",
+        "table": "dbo.jobs",
+        "grain": ["id"],
+        "columns": [
+            {"name": "id", "type": "number"},
+            {"name": "amount", "type": "number"},
+            {"name": "trade_name", "type": "string"},
+            {"name": "base", "type": "number"},
+            {"name": "doubled", "type": "number", "expr": "base * 2"},
+        ],
+        "segments": [
+            {"name": "is_roofing", "expr": "trade_name = 'Roofing'"},
+        ],
+        "measures": [
+            {
+                "name": "roofing_rev",
+                "expr": "sum(amount)",
+                "filter": "trade_name = 'Roofing'",
+            },
+        ],
+    }
+    base.update(overrides)
+    return base
+
+
+def _chasm_sources():
+    """Two facts fanning out from a shared hub — triggers aggregate locality, the
+    path that rewrites HAVING filters against measure references."""
+    fact = lambda name: {  # noqa: E731
+        "name": name,
+        "table": f"dbo.{name}",
+        "grain": ["id"],
+        "columns": [
+            {"name": "id", "type": "number"},
+            {"name": "hub_id", "type": "number"},
+            {"name": "val", "type": "number"},
+        ],
+        "joins": [
+            {"to": "hub", "on": "hub_id = hub.id", "relationship": "many_to_one"}
+        ],
+        "measures": [{"name": f"{name}_total", "expr": "sum(val)"}],
+    }
+    return {
+        "hub": {
+            "name": "hub",
+            "table": "dbo.hub",
+            "grain": ["id"],
+            "columns": [
+                {"name": "id", "type": "number"},
+                {"name": "segment", "type": "string"},
+            ],
+        },
+        "fact_a": fact("fact_a"),
+        "fact_b": fact("fact_b"),
+    }
+
+
+def _assert_valid(sql: str, dialect: str) -> None:
+    parsed = sqlglot.parse(sql, read=dialect)
+    assert parsed and all(stmt is not None for stmt in parsed), sql
+
+
+# Dialects whose grammar contains the `alias = expression` projection form
+# alongside ones that do not, to guard the cross-dialect contract.
+DIALECTS = ["tsql", "postgres", "snowflake", "bigquery"]
+
+
+@pytest.mark.parametrize("dialect", DIALECTS)
+def test_measure_equality_filter_compiles_as_comparison(dialect):
+    engine = make_engine({"jobs": _jobs_source()}, dialect=dialect)
+    sql = engine.query({"measures": ["jobs.roofing_rev"]}).sql
+
+    _assert_valid(sql, dialect)
+    assert "CASE WHEN" in sql.upper()
+    assert "'Roofing'" in sql
+    # The filter must remain an equality comparison, never an aliased literal.
+    assert "AS trade_name" not in sql
+    assert "'Roofing' AS" not in sql
+
+
+@pytest.mark.parametrize("dialect", DIALECTS)
+def test_segment_equality_filter_compiles_as_comparison(dialect):
+    engine = make_engine({"jobs": _jobs_source()}, dialect=dialect)
+    sql = engine.query(
+        {"measures": ["sum(jobs.amount)"], "segments": ["jobs.is_roofing"]}
+    ).sql
+
+    _assert_valid(sql, dialect)
+    assert "CASE WHEN" in sql.upper()
+    assert "'Roofing' AS" not in sql
+
+
+@pytest.mark.parametrize("dialect", DIALECTS)
+def test_where_equality_filter_compiles_as_comparison(dialect):
+    engine = make_engine({"jobs": _jobs_source()}, dialect=dialect)
+    sql = engine.query(
+        {"measures": ["sum(jobs.amount)"], "filters": ["jobs.trade_name = 'Roofing'"]}
+    ).sql
+
+    _assert_valid(sql, dialect)
+    assert "WHERE" in sql.upper()
+    assert "'Roofing' AS" not in sql
+
+
+@pytest.mark.parametrize("dialect", DIALECTS)
+def test_computed_column_expands_in_equality_where_filter(dialect):
+    engine = make_engine({"jobs": _jobs_source()}, dialect=dialect)
+    sql = engine.query(
+        {"measures": ["sum(jobs.amount)"], "filters": ["jobs.doubled = 10"]}
+    ).sql
+
+    _assert_valid(sql, dialect)
+    # `doubled` is computed (base * 2); the filter must reference the underlying
+    # expression, not the (non-existent) computed column name.
+    assert "base" in sql
+    assert "doubled" not in sql
+
+
+@pytest.mark.parametrize("dialect", DIALECTS)
+def test_locality_named_measure_equality_filter_compiles_as_comparison(dialect):
+    """Aggregate-locality HAVING filter on a named measure must compile to a
+    comparison, not the `0 AS measure` alias form T-SQL emits."""
+    engine = make_engine(_chasm_sources(), dialect=dialect)
+    sql = engine.query(
+        {
+            "measures": ["fact_a.fact_a_total", "fact_b.fact_b_total"],
+            "dimensions": ["hub.segment"],
+            "filters": ["fact_a.fact_a_total = 0"],
+        }
+    ).sql
+
+    _assert_valid(sql, dialect)
+    assert "0 AS fact_a_total" not in sql
+
+
+def test_tsql_equality_and_in_filters_are_equivalent_shape():
+    """On T-SQL, `= 'x'` and `IN ('x')` filters both compile to predicates."""
+    eq_engine = make_engine({"jobs": _jobs_source()}, dialect="tsql")
+    in_engine = make_engine(
+        {
+            "jobs": _jobs_source(
+                measures=[
+                    {
+                        "name": "roofing_rev",
+                        "expr": "sum(amount)",
+                        "filter": "trade_name IN ('Roofing')",
+                    }
+                ]
+            )
+        },
+        dialect="tsql",
+    )
+    eq_sql = eq_engine.query({"measures": ["jobs.roofing_rev"]}).sql
+    in_sql = in_engine.query({"measures": ["jobs.roofing_rev"]}).sql
+
+    _assert_valid(eq_sql, "tsql")
+    _assert_valid(in_sql, "tsql")
+    assert "jobs.trade_name = 'Roofing'" in eq_sql
+    assert "jobs.trade_name IN ('Roofing')" in in_sql