v1: legacy alignment warning fires on @/dot even when operands are exactly aligned

[FabianHofmann]

🤖 This issue was written by Claude (Claude Code), based on hands-on findings while migrating PyPSA to the v1 arithmetic semantics. Reproductions were verified against the PR #717 branch.

Summary

The legacy alignment LinopySemanticsWarning fires on @ / dot even when the operands are already exactly aligned (identical coordinate labels and order) on the contracted dimension. v1 accepts the same expression without complaint and both produce identical results, so the legacy warning appears to be a false positive for the aligned case.

Reproduction

linopy 0.8.0.post1.dev123+g5a131a628 (branch feat/arithmetic-convention), Python 3.12:

import warnings, numpy as np, pandas as pd, xarray as xr
import linopy as ln
from linopy.config import LinopySemanticsWarning

m = ln.Model()
names = pd.Index(["a", "b", "c"], name="name")
sns   = pd.Index([0, 1, 2], name="snapshot")
x = m.add_variables(coords=[sns, names], name="x")
C = xr.DataArray(np.ones((3, 2)), dims=["name", "cyc"],
                 coords={"name": names})          # same labels + order as x

for sem in ["legacy", "v1"]:
    ln.options["semantics"] = sem
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        x @ C
        warned = any(issubclass(a.category, LinopySemanticsWarning) for a in w)
        print(sem, "->", "WARN" if warned else "ok")

legacy -> WARN
v1     -> ok

The name coordinate is identical on both operands (same labels, same order); the contraction is unambiguous. v1 accepts it and the resulting expression matches legacy's.

Why it matters

This showed up in PyPSA's Kirchhoff voltage law constraint (flow @ C_branch). After I aligned the operands' name coordinates so v1 would accept the expression, v1 passed but legacy still warned at the same line — and with LinopySemanticsWarning subclassing FutureWarning, downstreams that treat FutureWarning as an error get churn on code that is already correct under both semantics.

Suggestion

Scope the legacy alignment warning on @ / dot (and, if applicable, the reductions that share this path) to actual divergence — i.e. don't warn when the operands are already exactly aligned on the shared/contracted dimension, since there is nothing for the caller to change.