From 626e867a9327e2233d48586698a402bb4658aaf9 Mon Sep 17 00:00:00 2001
From: Paul Dann <pgdann@gmail.com>
Date: Thu, 24 Jul 2025 15:58:52 +0100
Subject: [PATCH] Validate type of :deps_paths option for formatter_for_file/2

---
 lib/mix/lib/mix/tasks/format.ex | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/mix/lib/mix/tasks/format.ex b/lib/mix/lib/mix/tasks/format.ex
index 59b937e482..cbcddaea8f 100644
--- a/lib/mix/lib/mix/tasks/format.ex
+++ b/lib/mix/lib/mix/tasks/format.ex
@@ -518,6 +518,12 @@ defmodule Mix.Tasks.Format do
   defp eval_deps_opts(deps, opts) do
     deps_paths = opts[:deps_paths] || Mix.Project.deps_paths()
 
+    if not is_map(deps_paths) do
+      Mix.raise(
+        "Expected :deps_paths to return a map of dependency paths, got: #{inspect(deps_paths)}"
+      )
+    end
+
     for dep <- deps,
         dep_path = assert_valid_dep_and_fetch_path(dep, deps_paths),
         dep_dot_formatter = Path.join(dep_path, ".formatter.exs"),