fix(build): properly handle alias key in config merge

[chrissantamaria]

Description

👋 small bug fix - there's some logic in mergeConfig to handle merging particular field types:

vite/packages/vite/src/node/config.ts

Lines 520 to 539 in fa85749

	const existing = merged[key]
	if (Array.isArray(existing) && Array.isArray(value)) {
	merged[key] = [...existing, ...value]
	continue
	}
	if (isObject(existing) && isObject(value)) {
	merged[key] = mergeConfig(existing, value, false)
	continue
	}
	// root fields that require special handling
	if (existing != null && isRoot) {
	if (key === 'alias') {
	merged[key] = mergeAlias(existing, value)
	continue
	} else if (key === 'assetsInclude') {
	merged[key] = [].concat(existing, value)
	continue
	}
	}

Note the custom handling of alias at the bottom. In some cases this isn't necessary (i.e. if existing and value are both objects / arrays) though it is if the configs use different alias schemas.

It looks like mergeAlias is never being run since alias is (afaik) never a root key. I tweaked the logic to only perform this check when checking a non-root key.

I added a failing test for this case as well as an additional test for the assetsInclude (already passes on main).

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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[antfu]

VitePress's alias seems to be affected by this. Rising to P3.