[carljm]

A generic function that returns its argument can lose the caller’s type variable when its bound contains Any or Unknown. For example, ty inferred list[Any] for keep(items) here and reported unsound-return-statement when that rule was enabled. It now infers list[T].

from typing import Any

def keep[L: list[Any]](items: L) -> L:
    return items

def forward[T](items: list[T]) -> list[T]:
    return keep(items)

The argument gives the solver list[T] ≤ L, and the return annotation gives it L ≤ list[T]. The solver combines those constraints as L = list[T]. When it checks list[T] against the declared bound list[Any], its invariant comparison records T = Any. That comparison succeeds for every caller specialization of T. Using the recorded equality to rewrite the argument constraint gives list[Any] ≤ L.

The solver previously treated this last step as replacing the argument constraint, so it lost the evidence that the caller passed list[T]. When the replacement uses Any or Unknown from a constraint with declared provenance, ty now records the implication and keeps the original argument constraint available for inference.

Test plan

Compared the four returns in xarray’s _easy_facetgrid on main and with the sequent change alone. All four previously reported FacetGrid[Unknown] where the caller expected FacetGrid[T_DataArrayOrSet]. All four warnings clear with this change.

Ecosystem report

On 0f304c75, the only stable changes were four removed unsound-return-statement warnings in xarray. Each plotting method returns its receiver, and ty now preserves the grid’s data type on all four returns. Full ecosystem report

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Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 98.60%. The percentage of expected errors that received a diagnostic held steady at 98.86%. The number of fully passing files held steady at 137/146.

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Results for 0f304c7 · Workflow run

[astral-automations-bot]

Memory usage report

Memory usage unchanged ✅

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Results for 0f304c7 · Workflow run

[astral-automations-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
unsound-return-statement	0	4	0
Total	0	4	0

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Raw diff:

xarray (https://github.com/pydata/xarray)
- xarray/plot/facetgrid.py:1096:16 warning[unsound-return-statement] Unsound return statement: `FacetGrid[Unknown]` is not a subtype of `FacetGrid[T_DataArrayOrSet@_easy_facetgrid]`
- xarray/plot/facetgrid.py:1099:16 warning[unsound-return-statement] Unsound return statement: `FacetGrid[Unknown]` is not a subtype of `FacetGrid[T_DataArrayOrSet@_easy_facetgrid]`
- xarray/plot/facetgrid.py:1102:16 warning[unsound-return-statement] Unsound return statement: `FacetGrid[Unknown]` is not a subtype of `FacetGrid[T_DataArrayOrSet@_easy_facetgrid]`
- xarray/plot/facetgrid.py:1105:16 warning[unsound-return-statement] Unsound return statement: `FacetGrid[Unknown]` is not a subtype of `FacetGrid[T_DataArrayOrSet@_easy_facetgrid]`

Full report with detailed diff · Timing results

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Results for 0f304c7 · Workflow run

[carljm]

Ecosystem report

PR #30294 ecosystem summary

Four warnings disappeared from xarray’s plotting code. Each affected method returns the grid it was called on. ty now preserves that grid’s data type across the call. These were the only stable diagnostic changes.

Diagnostic changes (1 section)

1. xarray plotting methods preserve the grid’s data type

4 removed unsound-return-statement warnings

Report entries

• Removed unsound-return-statement on map_dataarray_line. facetgrid.py:1096
• Removed unsound-return-statement on map_dataarray. facetgrid.py:1099
• Removed unsound-return-statement on map_plot1d. facetgrid.py:1102
• Removed unsound-return-statement on map_dataset. facetgrid.py:1105

xarray’s _easy_facetgrid creates a FacetGrid[T] from the caller’s data, then returns the result of a plotting method. Each method annotates its receiver and return with the same type variable, whose bound is the bare FacetGrid. The missing type argument in that bound is Unknown. On the merge base, ty inferred FacetGrid[Unknown] for the return below and warned that it could not guarantee FacetGrid[T]. The PR retains the receiver’s FacetGrid[T], so the return satisfies the annotation. xarray receiver bound

class FacetGrid[T]:
    data: T

    def map_dataarray_line[G: FacetGrid](self: G) -> G:
        return self


def _easy_facetgrid[T](g: FacetGrid[T]) -> FacetGrid[T]:
    return g.map_dataarray_line()

Reproduction

Versions and settings

• Compared base 6e204755 with tested merge 1f58294c for PR head 0f304c75. Actions run 42946012957, attempt 1 · Detailed report
• Used ecosystem-analyzer 96671445 and mypy-primer 4bec1e88 with dependency cutoff 2026-11-11T02:26:09Z.
• Ran the CI profiling binaries in an Ubuntu 24.04 x86-64 container against xarray’s Python 3.11 environment with a Linux target, uv 0.13.7, TY_UV=scripts, UV_LOCKED=1, and RUST_BACKTRACE=1. The analyzer’s dev profile allows 180 seconds per check.
• Checked xarray in strict mode with ty check . --python .venv --output-format concise --config analysis.strict-equality-semantics=true --config analysis.strict-generic-narrowing=true. The full-project diagnostic count went from 2,983 to 2,979.