feat(is_dst): real per-backend implementation (item 65)

docs/reference/expression-coverage-scoped.md

@@ -5,7 +5,7 @@
 
 Scoped deviations — dialect, parameter, option, value-class; function-level coverage and matrices live in [`expression-coverage.md`](expression-coverage.md).
 
-Declarations: 42 · Facts: 1558 · Registered operations: 324 · Implementation records: 972
+Declarations: 42 · Facts: 1560 · Registered operations: 324 · Implementation records: 972
 
 Legend — scoped deviations:
 
@@ -165,6 +165,13 @@ Legend — scoped deviations:
 | ibis-duckdb | unit | 1q | — | unsupported | gate | build | — | ibis TimestampTruncate rejects the quarter unit '1q' (and its friendly alias 'quarter') | — | — | 2026-07-24 | — | — |
 | ibis-duckdb | unit | quarter | — | unsupported | gate | build | — | ibis TimestampTruncate rejects the quarter unit '1q' (and its friendly alias 'quarter') | — | — | 2026-07-24 | — | — |
 
+### `IS_DST` × ibis (FKEY_MOUNTAINASH_SCALAR_DATETIME)
+
+| Dialect | Param | Option values | Value class | Level | Enforcement | Boundary | Condition | Message | Workaround | Upstream | Since | Native errors | Probe-exempt |
+| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
+| * | timezone | — | iana_timezone | unsupported | gate | build | — | is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb) | — | — | 2026-08-16 | — | — |
+| ibis-duckdb | timezone | — | iana_timezone | unsupported | gate | build | — | is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb) | — | — | 2026-08-16 | — | — |
+
 ### `ROUND` × narwhals (FKEY_MOUNTAINASH_SCALAR_DATETIME)
 
 | Dialect | Param | Option values | Value class | Level | Enforcement | Boundary | Condition | Message | Workaround | Upstream | Since | Native errors | Probe-exempt |

docs/reference/expression-coverage.json

@@ -1,7 +1,7 @@
 {
   "stamp": {
     "declarations": 42,
-    "facts": 1558,
+    "facts": 1560,
     "operations": 324,
     "implementation_records": 972
   },
@@ -39,8 +39,8 @@
           "unknown": 0
         },
         "default_capable": 138,
-        "audited_clean": 112,
-        "constrained": 74,
+        "audited_clean": 111,
+        "constrained": 75,
         "audited_unknown": 0
       }
     },
@@ -50,19 +50,19 @@
       "expr_capable": 149,
       "literal_only": 65,
       "polymorphic": 9,
-      "unsupported": 1335
+      "unsupported": 1337
     },
     "facts_by_enforcement": {
-      "gate": 1551,
+      "gate": 1553,
       "router_metadata": 3,
       "materialize_residue": 4
     },
     "facts_by_backend": {
       "polars": 265,
       "narwhals": 662,
-      "ibis": 631
+      "ibis": 633
     },
-    "facts_total": 1558
+    "facts_total": 1560
   },
   "families": [
     {
@@ -4997,15 +4997,48 @@
               "impl_protocol": "MountainAshIbisScalarDatetimeExpressionSystem",
               "audited": true,
               "whole_op": null,
-              "constrained": false,
+              "constrained": true,
               "contradiction": false,
               "selector_counts": {
-                "params": 0,
+                "params": 1,
                 "option_selectors": 0,
-                "value_classes": 0,
-                "dialects": 0
+                "value_classes": 1,
+                "dialects": 1
               },
-              "constraints": [],
+              "constraints": [
+                {
+                  "dialect": null,
+                  "param": "timezone",
+                  "option_value": null,
+                  "value_class": "iana_timezone",
+                  "level": "unsupported",
+                  "enforcement": "gate",
+                  "boundary": "build",
+                  "condition": null,
+                  "message": "is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb)",
+                  "workaround": null,
+                  "upstream_ref": null,
+                  "since": "2026-08-16",
+                  "native_errors": [],
+                  "probe_exempt": null
+                },
+                {
+                  "dialect": "ibis-duckdb",
+                  "param": "timezone",
+                  "option_value": null,
+                  "value_class": "iana_timezone",
+                  "level": "unsupported",
+                  "enforcement": "gate",
+                  "boundary": "build",
+                  "condition": null,
+                  "message": "is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb)",
+                  "workaround": null,
+                  "upstream_ref": null,
+                  "since": "2026-08-16",
+                  "native_errors": [],
+                  "probe_exempt": null
+                }
+              ],
               "residue": [],
               "routed": [],
               "refinements": []
@@ -48140,6 +48173,22 @@
           "native_errors": [],
           "probe_exempt": null
         },
+        {
+          "dialect": null,
+          "param": "timezone",
+          "option_value": null,
+          "value_class": "iana_timezone",
+          "level": "unsupported",
+          "enforcement": "gate",
+          "boundary": "build",
+          "condition": null,
+          "message": "is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb)",
+          "workaround": null,
+          "upstream_ref": null,
+          "since": "2026-08-16",
+          "native_errors": [],
+          "probe_exempt": null
+        },
         {
           "dialect": null,
           "param": "unit",
@@ -48220,6 +48269,22 @@
           "native_errors": [],
           "probe_exempt": null
         },
+        {
+          "dialect": "ibis-duckdb",
+          "param": "timezone",
+          "option_value": null,
+          "value_class": "iana_timezone",
+          "level": "unsupported",
+          "enforcement": "gate",
+          "boundary": "build",
+          "condition": null,
+          "message": "is_dst is not supported on ibis -- ibis has no DST/timezone-offset primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb)",
+          "workaround": null,
+          "upstream_ref": null,
+          "since": "2026-08-16",
+          "native_errors": [],
+          "probe_exempt": null
+        },
         {
           "dialect": "ibis-duckdb",
           "param": "unit",

docs/reference/expression-coverage.md

@@ -3,7 +3,7 @@
 <!-- GENERATED FILE — do not edit by hand. -->
 <!-- Regenerate: hatch -e test run python -m mountainash.core.capabilities.render_markdown -->
 
-Declarations: 42 · Facts: 1558 · Registered operations: 324 · Implementation records: 972
+Declarations: 42 · Facts: 1560 · Registered operations: 324 · Implementation records: 972
 
 Scoped deviations (dialect/param/option/value-class) live in [`expression-coverage-scoped.md`](expression-coverage-scoped.md).
 
@@ -52,7 +52,7 @@ Legend — cell states (by exception):
 | --- | --- | --- | --- | --- | --- | --- |
 | polars | 191 | 81 | 52 | 0 | 0 | 324 |
 | narwhals | 97 | 149 | 78 | 0 | 0 | 324 |
-| ibis | 138 | 112 | 74 | 0 | 0 | 324 |
+| ibis | 138 | 111 | 75 | 0 | 0 | 324 |
 
 contradictions: 0
 audited_unknown: 0
@@ -61,9 +61,9 @@ audited_unknown: 0
 
 | Axis | Breakdown |
 | --- | --- |
-| Level | expr_capable 149, literal_only 65, polymorphic 9, unsupported 1335 |
-| Enforcement | gate 1551, router_metadata 3, materialize_residue 4 |
-| Backend | polars 265, narwhals 662, ibis 631 |
+| Level | expr_capable 149, literal_only 65, polymorphic 9, unsupported 1337 |
+| Enforcement | gate 1553, router_metadata 3, materialize_residue 4 |
+| Backend | polars 265, narwhals 662, ibis 633 |
 
 `pandas` / `pyarrow` are routed input types (they execute via the narwhals path) and are not independent coverage columns.
 
@@ -161,7 +161,7 @@ audited_unknown: 0
 | `EXTRACT_WEEKDAY` | ✓ | ✓ audited | ✓ audited |
 | `EXTRACT_YEAR` | ✓ | ✓ audited | ✓ audited |
 | `FLOOR` | ✓ | ◐ partial (1 params, 2 option-selectors, 0 value-classes, 2 dialects) | ◐ partial (1 params, 2 option-selectors, 1 value-classes, 1 dialects) |
-| `IS_DST` | ✓ | ✓ audited | ✓ audited |
+| `IS_DST` | ✓ | ✓ audited | ◐ partial (1 params, 0 option-selectors, 1 value-classes, 1 dialects) |
 | `IS_LEAP_YEAR` | ✓ | ✓ audited | ✓ audited |
 | `MONTH_END` | ✓ | ✓ audited | ✓ audited |
 | `MONTH_START` | ✓ | ✓ audited | ✓ audited |

src/mountainash/expressions/backends/capabilities/datetime/value_classes_ma.py

@@ -80,6 +80,10 @@
     "bare CAST AS TIMESTAMPTZ), so any expression composed on the result "
     "raises UnsupportedOperationError (verified 2026-07-29, ibis 12.0.0/duckdb)"
 )
+_IS_DST_MSG = (
+    "is_dst is not supported on ibis -- ibis has no DST/timezone-offset "
+    "primitive to build on (verified 2026-08-16, ibis 12.0.0/duckdb)"
+)
 
 
 def _mult_fact(op: str, backend, dialect: str | None, message: str) -> CapabilityFact:
@@ -108,13 +112,29 @@ def _tz_fact(backend, dialect: str | None, message: str) -> CapabilityFact:
     )
 
 
+def _is_dst_fact(backend, dialect: str | None, message: str) -> CapabilityFact:
+    return CapabilityFact(
+        operation_key=FK_DT.IS_DST,
+        param="timezone",
+        value_class=ValueClass.IANA_TIMEZONE,
+        level=CapabilityLevel.UNSUPPORTED,
+        backend=backend,
+        dialect=dialect,
+        message=message,
+        since="2026-08-16",
+    )
+
+
 _IBIS_FACTS = tuple(
     _mult_fact(op, CONST_BACKEND.IBIS, dialect, _IBIS_MSG)
     for op in _IBIS_MULTIPLIER_OPS
     for dialect in (None, "ibis-duckdb")  # family default + duckdb
 ) + tuple(
     _tz_fact(CONST_BACKEND.IBIS, dialect, _TO_TIMEZONE_MSG)
     for dialect in (None, "ibis-duckdb")
+) + tuple(
+    _is_dst_fact(CONST_BACKEND.IBIS, dialect, _IS_DST_MSG)
+    for dialect in (None, "ibis-duckdb")
 )
 _NARWHALS_FACTS = tuple(
     _mult_fact(op, CONST_BACKEND.NARWHALS, dialect, _NARWHALS_ROUND_CEIL_MSG)

src/mountainash/expressions/backends/expression_systems/ibis/extensions_mountainash/expsys_ib_ext_ma_scalar_datetime.py

@@ -6,7 +6,7 @@
 from __future__ import annotations
 
 from datetime import date
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 import ibis
 
@@ -24,6 +24,10 @@
     "to_timezone is not supported on ibis — the target zone never reaches the "
     "engine; see capabilities/datetime/value_classes_ma.py"
 )
+_IS_DST_UNSUPPORTED = (
+    "is_dst is not supported on ibis — ibis has no DST/timezone-offset "
+    "primitive; see capabilities/datetime/value_classes_ma.py"
+)
 
 
 # Ibis .truncate() expects a bare unit letter, not the Polars-style "<n><unit>" duration.
@@ -143,14 +147,19 @@ def is_dst(
         self,
         x: IbisTemporalExpr,
         /,
-        timezone: Optional[str] = None,
+        timezone: str,
     ) -> IbisValueExpr:
-        """Check if DST is observed at this time.
+        """Ibis has no timezone/DST detection primitive; declared UNSUPPORTED.
 
-        Note: Ibis doesn't have direct DST detection.
-        Returns False as a placeholder.
+        Declared UNSUPPORTED on ibis (see capabilities/datetime/value_classes_ma.py)
+        — the capability gate raises before this method is reached. The raise
+        here is defence in depth (mirrors to_timezone's established pattern).
         """
-        return ibis.literal(False)
+        raise BackendCapabilityError(
+            _IS_DST_UNSUPPORTED,
+            backend="ibis",
+            function_key=FKEY_MOUNTAINASH_SCALAR_DATETIME.IS_DST,
+        )
 
     # =========================================================================
     # Date Arithmetic Methods

src/mountainash/expressions/backends/expression_systems/narwhals/extensions_mountainash/expsys_nw_ext_ma_scalar_datetime.py

@@ -5,8 +5,9 @@
 
 from __future__ import annotations
 
+import datetime as dt
 from datetime import date, datetime
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 import narwhals as nw
 
@@ -121,14 +122,43 @@ def is_dst(
         self,
         x: NarwhalsExpr,
         /,
-        timezone: Optional[str] = None,
+        timezone: str,
     ) -> NarwhalsExpr:
-        """Check if DST is observed at this time.
-
-        Note: Narwhals doesn't have direct DST detection.
-        Returns False as a placeholder.
+        """Return True if DST is observed for x in the given timezone.
+
+        Narwhals has no direct DST/offset primitive. Hand-rolled via two
+        reference-offset comparisons: compute the zone's UTC offset at `x`,
+        and compare against the smaller (standard-time) of two fixed
+        reference-instant offsets six months apart. DST always moves the
+        offset in the direction that makes local time further from UTC, so
+        `current_offset > standard_offset` <=> DST is in effect. Zones with
+        no DST have equal reference offsets, so this is always False.
+
+        Verified against `zoneinfo` for UTC, America/New_York,
+        America/Los_Angeles, Europe/Paris, Australia/Sydney,
+        Pacific/Auckland, Asia/Kolkata (both hemispheres).
+
+        Warning: `utc_naive` must always be derived directly from `x`
+        (never by converting back from `local_naive`), and every reference
+        offset must be derived directly from a fresh UTC-anchored literal --
+        narwhals 2.24.0 + pandas 3.0.5 raises AttributeError when a second
+        `convert_time_zone` call chains directly onto an expression whose
+        first hop already targeted America/New_York or America/Los_Angeles.
         """
-        return nw.lit(False)
+        utc_naive = x.dt.convert_time_zone("UTC").dt.replace_time_zone(None)
+        local_naive = x.dt.convert_time_zone(timezone).dt.replace_time_zone(None)
+        current_offset = local_naive - utc_naive
+
+        def _offset_at(utc_dt: datetime) -> NarwhalsExpr:
+            ref_utc = nw.lit(utc_dt)
+            ref_utc_naive = ref_utc.dt.replace_time_zone(None)
+            ref_local_naive = ref_utc.dt.convert_time_zone(timezone).dt.replace_time_zone(None)
+            return ref_local_naive - ref_utc_naive
+
+        ref1 = _offset_at(datetime(2020, 1, 1, 12, 0, tzinfo=dt.timezone.utc))
+        ref2 = _offset_at(datetime(2020, 7, 1, 12, 0, tzinfo=dt.timezone.utc))
+        standard_offset = nw.when(ref1 < ref2).then(ref1).otherwise(ref2)
+        return current_offset > standard_offset
 
     # =========================================================================
     # Date Arithmetic Methods

src/mountainash/expressions/backends/expression_systems/polars/extensions_mountainash/expsys_pl_ext_ma_scalar_datetime.py

@@ -6,7 +6,7 @@
 from __future__ import annotations
 
 from datetime import date, datetime
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 import polars as pl
 
@@ -116,14 +116,25 @@ def is_dst(
         self,
         x: PolarsExpr,
         /,
-        timezone: Optional[str] = None,
+        timezone: str,
     ) -> PolarsExpr:
-        """Check if DST is observed at this time.
+        """Return True if DST is observed for x in the given timezone.
 
-        Note: Polars doesn't have direct DST detection.
-        Returns False as a placeholder.
+        Args:
+            x: Datetime expression.
+            timezone: IANA timezone name. `x` need not be timezone-aware:
+                `convert_time_zone` converts to the target zone.
+
+        Returns:
+            Boolean expression; True where DST is in effect. Matches the
+            existing extract/extract_boolean precedent: a naive `x` is
+            silently treated as UTC (no guard applied here either).
         """
-        return pl.lit(False)
+        return (
+            x.dt.convert_time_zone(timezone)
+            .dt.dst_offset()
+            .ne(pl.duration(seconds=0))
+        )
 
     # =========================================================================
     # Date Arithmetic Methods