Facts

The appellant, the Commune of Jorat-Mézières (hereinafter: the Commune), brings a subsidiary constitutional appeal against the judgment of 9 December 2019 of the Administrative and Public Law Division of the Cantonal Court of the Canton of Vaud (hereinafter: the Cantonal Court), which allowed the appeal of the respondent [A.] and annulled the decision of the Municipality of Jorat-Mézières (hereinafter: the Municipality) of 13 May 2019 refusing to grant her communal citizenship. The matter was remitted to the Municipality for further investigation and a new decision.

The respondent, an Iranian national born in 1972, arrived in Switzerland in 2002 and has resided in Carrouge (Commune of Jorat-Mézières) since 2012. She holds a settlement permit and has been separated from her husband since 2015. She filed an application for ordinary naturalisation on 22 January 2018. The investigation report of the Population Service of the Canton of Vaud (SPOP) refers to training as a nursing assistant, internships, employment as a housekeeper from 2007 to 2016, total incapacity for work since 1 January 2017 and a disability insurance pension. She has not been subject to any sanction for threats to security or public order.

An extract from the debt enforcement register dated 3 December 2018 records a debt enforcement notified on 27 February 2018, to which an objection was raised, brought by her husband for 600'000 francs. No statement of insolvency has been drawn up. The respondent passed the test of elementary knowledge (44 points out of 48) and was heard by the Naturalisation Commission on 20 February 2019. The Municipality issued a negative preliminary opinion on 26 February 2019, followed by the SPOP on 7 March 2019. In her submissions of 17 April 2019, the respondent explained that the debt enforcement concerned an unjustified claim by her spouse relating to a donation of a share of co-ownership.

By decision of 13 May 2019, the Municipality refused naturalisation on account of the debt enforcement subject to an objection (art. 12 let. a SCA). The Cantonal Court annulled this decision and remitted the matter for further investigation.

Before the Federal Supreme Court, the Commune seeks confirmation of the decision of 13 May 2019. The Cantonal Court waived submitting observations, the SPOP implicitly seeks dismissal, and the respondent seeks inadmissibility or, in the alternative, dismissal. The presiding Judge granted suspensive effect by order of 18 February 2020.

Considerations

An appeal in public law matters is inadmissible against decisions relating to ordinary naturalisation (art. 83 let. b FSCA), so that the subsidiary constitutional appeal (art. 113 FSCA) is open. The FSCA does not regulate the standing of a public body to appeal in this context. According to art. 115 FSCA, a party must have taken part in the proceedings below and have a legal interest in the annulment or amendment of the decision, which presupposes being the holder of a constitutional right invoked. In principle, public bodies acting as holders of public authority do not enjoy constitutional rights, but case law admits an exception where they invoke guarantees such as their autonomy (ATF 145 I 239 consid. 5.1).

The Commune, which invokes the violation of its autonomy in matters of granting communal citizenship, has standing to act; whether it is in fact autonomous in this area is a question of the merits. It may also rely, on that basis, on the prohibition of arbitrariness and the right to be heard.

The cantonal judgment annuls the municipal decision and remits the matter for a new decision. It does not terminate the cantonal proceedings and therefore does not constitute a final decision (art. 90 FSCA), but an incidental or preliminary decision. It may be challenged immediately only if it causes irreparable harm (art. 93 para. 1 let. a FSCA) or if the admission of the appeal may immediately lead to a final decision avoiding lengthy and costly evidentiary proceedings (art. 93 para. 1 let.…