Move `At.member` from `GroupMessage` to `MessagePacket`

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

@@ -14,7 +14,6 @@ import net.mamoe.mirai.contact.Group
 import net.mamoe.mirai.contact.Member
 import net.mamoe.mirai.contact.MemberPermission
 import net.mamoe.mirai.event.Event
-import net.mamoe.mirai.message.data.At
 import net.mamoe.mirai.message.data.Message
 import net.mamoe.mirai.message.data.MessageChain
 import net.mamoe.mirai.utils.getValue
@@ -38,7 +37,6 @@ class GroupMessage(
 
     override val subject: Group get() = group
 
-    inline fun At.member(): Member = group[this.target]
     inline fun Long.member(): Member = group[this]
 
 

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

@@ -115,6 +115,8 @@ abstract class MessagePacketBase<TSender : QQ, TSubject : Contact>(_bot: Bot) :
      */
     inline fun QQ.at(): At = At(this as? Member ?: error("`QQ.at` can only be used in GroupMessage"))
 
+    inline fun At.member(): Member = (this@MessagePacketBase as? GroupMessage)?.group?.get(this.target) ?: error("`At.member` can only be used in GroupMessage")
+
     // endregion
 
     // region 下载图片