Improve typing of resolve metadata

packages/next/src/lib/metadata/generate/utils.ts

@@ -1,20 +1,17 @@
-function resolveArray<T>(value: T): T[] {
+function resolveArray<T>(value: T | T[]): T[] {
   if (Array.isArray(value)) {
-    return value
+    return value as any
   }
-  return [value]
+  return [value] as any
 }
 
-function resolveAsArrayOrUndefined<T extends unknown | readonly unknown[]>(
+function resolveAsArrayOrUndefined<T>(
   value: T | T[] | undefined | null
-): undefined | T[] {
+): T extends undefined | null ? undefined : T[] {
   if (typeof value === 'undefined' || value === null) {
-    return undefined
+    return undefined as any
   }
-  if (Array.isArray(value)) {
-    return value
-  }
-  return [value]
+  return resolveArray(value) as any
 }
 
 export { resolveAsArrayOrUndefined, resolveArray }

packages/next/src/lib/metadata/resolve-metadata.ts

@@ -161,11 +161,12 @@ function merge({
       case 'archives':
       case 'assets':
       case 'bookmarks':
-      case 'keywords':
+      case 'keywords': {
+        target[key] = resolveAsArrayOrUndefined(source[key])
+        break
+      }
       case 'authors': {
-        // FIXME: type inferring
-        // @ts-ignore
-        target[key] = resolveAsArrayOrUndefined(source[key]) || null
+        target[key] = resolveAsArrayOrUndefined(source.authors)
         break
       }
       // directly assign fields that fallback to null

packages/next/src/lib/metadata/resolvers/resolve-basics.ts

@@ -83,7 +83,7 @@ function resolveUrlValuesOfObject(
     if (typeof value === 'string' || value instanceof URL) {
       result[key] = [
         {
-          url: resolveAlternateUrl(value, metadataBase, pathname), // metadataBase ? resolveUrl(value, metadataBase)! : value,
+          url: resolveAlternateUrl(value, metadataBase, pathname),
         },
       ]
     } else {