Respect dropped (but explicit) indexes in dependency groups (#17012)

## Summary

There are a class of outcomes whereby an index might not be included in
"allowed indexes", but could still correctly appear in a lockfile. In
the linked case, we have two `default = true` indexes, and one of them
is also named. We omit the second `default = true` index from the list
of "allowed indexes", but since it's named, a dependency can reference
it explicitly. We handle this correctly for `project.dependencies`, but
the handling was incorrectly omitting dependency groups.

Closes https://github.com/astral-sh/uv/issues/16843.

crates/uv-resolver/src/lock/mod.rs

@@ -1969,7 +1969,12 @@ impl Lock {
             // not be available as top-level configuration (i.e., if they're defined within a
             // workspace member), but we already validated that the dependencies are up-to-date, so
             // we can consider them "available".
-            for requirement in &package.metadata.requires_dist {
+            for requirement in package
+                .metadata
+                .requires_dist
+                .iter()
+                .chain(package.metadata.dependency_groups.values().flatten())
+            {
                 if let RequirementSource::Registry {
                     index: Some(index), ..
                 } = &requirement.source

crates/uv/tests/it/lock.rs

@@ -32281,3 +32281,100 @@ fn lock_unsupported_wheel_url_required_platform() -> Result<()> {
 
     Ok(())
 }
+
+/// If an index is filtered out (e.g., it's the second `default = true` index defined in the file),
+/// we should still consider the lockfile valid if it's referenced by name, regardless of whether
+/// it's defined in a dependency group or the top-level `project.dependencies` field.
+///
+/// See: <https://github.com/astral-sh/uv/issues/16843>
+#[test]
+fn lock_check_multiple_default_indexes_explicit_assignment_dependency_group() -> Result<()> {
+    let context = TestContext::new("3.12").with_exclude_newer("2025-01-30T00:00:00Z");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+
+        [dependency-groups]
+        dev = ["iniconfig"]
+
+        [tool.uv.sources]
+        iniconfig = { index = "second" }
+
+        [[tool.uv.index]]
+        name = "first"
+        url = "https://test.pypi.org/simple"
+        default = true
+
+        [[tool.uv.index]]
+        name = "second"
+        url = "https://public:heron@pypi-proxy.fly.dev/basic-auth/simple"
+        default = true
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    ");
+
+    let lock = context.read("uv.lock");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            lock, @r#"
+        version = 1
+        revision = 3
+        requires-python = ">=3.12"
+
+        [options]
+        exclude-newer = "2025-01-30T00:00:00Z"
+
+        [[package]]
+        name = "iniconfig"
+        version = "2.0.0"
+        source = { registry = "https://pypi-proxy.fly.dev/basic-auth/simple" }
+        sdist = { url = "https://pypi-proxy.fly.dev/basic-auth/files/packages/d7/4b/cbd8e699e64a6f16ca3a8220661b5f83792b3017d0f79807cb8708d33913/iniconfig-2.0.0.tar.gz", hash = "sha256:2d91e135bf72d31a410b17c16da610a82cb55f6b0477d1a902134b24a455b8b3", size = 4646, upload-time = "2023-01-07T11:08:11.254Z" }
+        wheels = [
+            { url = "https://pypi-proxy.fly.dev/basic-auth/files/packages/ef/a6/62565a6e1cf69e10f5727360368e451d4b7f58beeac6173dc9db836a5b46/iniconfig-2.0.0-py3-none-any.whl", hash = "sha256:b6a85871a79d2e3b22d2d1b94ac2824226a63c6b741c88f7ae975f18b6778374", size = 5892, upload-time = "2023-01-07T11:08:09.864Z" },
+        ]
+
+        [[package]]
+        name = "project"
+        version = "0.1.0"
+        source = { virtual = "." }
+
+        [package.dev-dependencies]
+        dev = [
+            { name = "iniconfig" },
+        ]
+
+        [package.metadata]
+
+        [package.metadata.requires-dev]
+        dev = [{ name = "iniconfig", index = "https://pypi-proxy.fly.dev/basic-auth/simple" }]
+        "#
+        );
+    });
+
+    uv_snapshot!(context.filters(), context.lock().arg("--check"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    ");
+
+    Ok(())
+}