Add space automatically if necessary

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/MessagePacket.kt

@@ -92,7 +92,7 @@ abstract class MessagePacketBase<TSender : QQ, TSubject : Contact>(_bot: Bot) :
     // region
 
     /**
-     * 引用这个消息.
+     * 引用这个消息. 当且仅当消息为群消息时可用. 否则将会抛出 [IllegalArgumentException]
      */
     inline fun MessageChain.quote(): MessageChain = this.quote(sender as? Member ?: error("only group message can be quoted"))
 

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/At.kt

@@ -30,6 +30,15 @@ class At @MiraiInternalAPI constructor(val target: Long, val display: String) :
     override fun eq(other: Message): Boolean {
         return other is At && other.target == this.target
     }
+
+    // 自动为消息补充 " "
+
+    override fun followedBy(tail: Message): MessageChain {
+        if(tail is PlainText && !tail.stringValue.startsWith(' ')){
+            return super.followedBy(PlainText(" ")) + tail
+        }
+        return super.followedBy(tail)
+    }
 }
 
 /**