Cascade array settings not merged from imported module

[doompadee]

What version of Hugo are you using (hugo version)?

hugo v0.148.1-98ba786f2f5dca0866f47ab79f394370bcb77d2f+extended linux/amd64 BuildDate=2025-07-11T12:56:21Z VendorInfo=gohugoio

Does this issue reproduce with the latest release?

Yes

Importing a module with cascade array settings does not merge the settings with the main config, even if the main config says so via '_merge=deep' (if I understand it correctly, technically '_merge' should not be necessary here, as there are no other cascade settings, but I could be wrong).

I've created a showcase repo here: https://github.com/doompadee/hugo-cascade

I would expect that ''hugo config' reveals the imported cascade settings. There are none at all when _merge is missing. With merge = deep there are only the section identifiers present, but no settings values.

See also: #12899

Reference: https://discourse.gohugo.io/t/unable-to-merge-cascade-config-from-theme-module/55409

[jmooring]

func TestIssue13869(t *testing.T) {
	t.Parallel()

	files := `
-- hugo.toml --
disableKinds = ['page','rss','section','sitemap','taxonomy','term']
theme = 'foo'
[cascade]
_merge = 'deep'
-- content/_index.md --
---
title: home
---
-- layouts/home.html --
color: {{ .Params.color }}
-- themes/foo/hugo.toml --
[[cascade]]
[cascade.params]
color = 'red'
`

	b := hugolib.Test(t, files)

	b.AssertFileContent("public/index.html", "color: red")
}

Merging the cascade configuration from a theme fails when the theme's cascade config is an array, but it works correctly when the config is a map. For example:

-- themes/foo/hugo.toml --
[cascade]
[cascade.params]
color = 'red'