laravel パッケージインストール時にエラー「Your requirements could not be resolved to an installable set of packages.」

Question

前提・実現したいこと

laravel5.8 の環境にdebugbar をインストールしようとしたところ件名のエラーが表示されました。
どう対処を行えばよいかアドバイスをいただけないでしょうか。
参考サイト

試したこと

php -d memory_limit=-1 を追加し、インストールを試みましたがダメでした。
参考サイト

「php -d memory_limit=-1 composer.phar require barryvdh/laravel-debugbar:~2 --dev」での実行をしましたが、エラーが発生しました。
(下記のエラーッセージとは別のエラーでしたが、文字制限のため、下記のエラーメッセージを削除しても掲載することが出来ませんでした。)

発生している問題・エラーメッセージ

% php -d memory_limit=-1 composer.phar require barryvdh/laravel-debugbar:~2
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.0
    - Conclusion: don't install barryvdh/laravel-debugbar v2.3.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.3.1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.3.0
    - Conclusion: don't install barryvdh/laravel-debugbar V2.2.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.2.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.2.1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.2.0
    - Conclusion: don't install barryvdh/laravel-debugbar v2.1.4
    - Conclusion: don't install barryvdh/laravel-debugbar v2.1.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.1.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.1.1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.1.0
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.6
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.5
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.4
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.1
    - Conclusion: don't install barryvdh/laravel-debugbar v2.0.0
    - Conclusion: remove laravel/framework v5.8.38
    - Installation request for barryvdh/laravel-debugbar ~2 -> satisfiable by barryvdh/laravel-debugbar[2.4.x-dev, V2.2.3, v2.0.0, v2.0.1, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.1.0, v2.1.1, v2.1.2, v2.1.3, v2.1.4, v2.2.0, v2.2.1, v2.2.2, v2.3.0, v2.3.1, v2.3.2, v2.4.0, v2.4.1, v2.4.2, v2.4.3].
    - Conclusion: don't install laravel/framework v5.8.38
    - barryvdh/laravel-debugbar 2.4.x-dev requires illuminate/support 5.1.*|5.2.*|5.3.*|5.4.*|5.5.* -> satisfiable by laravel/framework[5.1.x-dev, 5.2.x-dev, 5.3.x-dev, 5.4.x-dev, 5.5.x-dev], illuminate/support[5.1.x-dev, 5.2.x-dev, 5.3.x-dev, 5.4.x-dev, 5.5.x-dev, v5.1.1, v5.1.13, v5.1.16, v5.1.2, v5.1.20, v5.1.22, v5.1.25, v5.1.28, v5.1.30, v5.1.31, v5.1.41, v5.1.6, v5.1.8, v5.2.0, v5.2.19, v5.2.21, v5.2.24, v5.2.25, v5.2.26, v5.2.27, v5.2.28, v5.2.31, v5.2.32, v5.2.37, v5.2.43, v5.2.45, v5.2.6, v5.2.7, v5.3.0, v5.3.16, v5.3.23, v5.3.4, v5.4.0, v5.4.13, v5.4.17, v5.4.19, v5.4.27, v5.4.36, v5.4.9, v5.5.0, v5.5.16, v5.5.17, v5.5.2, v5.5.28, v5.5.33, v5.5.34, v5.5.35, v5.5.36, v5.5.37, v5.5.39, v5.5.40, v5.5.41, v5.5.43, v5.5.44].
    - Can only install one of: laravel/framework[5.1.x-dev, v5.8.38].
    - Can only install one of: laravel/framework[5.2.x-dev, v5.8.38].
    - Can only install one of: laravel/framework[5.3.x-dev, v5.8.38].
    - Can only install one of: laravel/framework[5.4.x-dev, v5.8.38].
    - Can only install one of: laravel/framework[5.5.x-dev, v5.8.38].
    - don't install illuminate/support 5.1.x-dev|don't install laravel/framework v5.8.38
    - don't install illuminate/support 5.2.x-dev|don't install laravel/framework v5.8.38
    - don't install illuminate/support 5.3.x-dev|don't install laravel/framework v5.8.38
    - don't install illuminate/support 5.4.x-dev|don't install laravel/framework v5.8.38
    - don't install illuminate/support 5.5.x-dev|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.1|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.13|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.16|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.2|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.20|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.22|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.25|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.28|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.30|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.31|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.41|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.6|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.1.8|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.0|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.19|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.21|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.24|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.25|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.26|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.27|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.28|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.31|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.32|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.37|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.43|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.45|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.6|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.2.7|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.3.0|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.3.16|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.3.23|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.3.4|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.0|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.13|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.17|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.19|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.27|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.36|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.4.9|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.0|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.16|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.17|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.2|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.28|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.33|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.34|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.35|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.36|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.37|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.39|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.40|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.41|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.43|don't install laravel/framework v5.8.38
    - don't install illuminate/support v5.5.44|don't install laravel/framework v5.8.38
    - Installation request for laravel/framework (locked at v5.8.38, required as 5.8.*) -> satisfiable by laravel/framework[v5.8.38].


Installation failed, reverting ./composer.json to its original content.

Answer 1

laravel-debugbar:~2の:~2が余計。

まず見る場所はGitHubしかない。
https://github.com/barryvdh/laravel-debugbar
composer.jsonを見れば対応してるLaravelのバージョンが分かるので
自分の使ってるLaravelのバージョンに合ったlaravel-debugbarを使う。
L5.8なら3.4.*と指定するしかなさそう。
何も余計なことせずにインストールすればcomposerが上手くやってくれるはずだけど。
インストール可能なバージョンをしっかり選んでくれる。

パッケージ開発者視点ではサポート期間はLaravelと同じなのでL5.8はすでに対象外。
3.4は"^5.5|^6|^7",一つ前のLTSであるL5.5をサポート対象とするため。
3.5は"^6|^7|^8",8月でL5.5のサポートが終了したので今のLTSである6以上が対象。
3.4->3.5の0.1で容赦なく旧バージョンを切るのはサポート切れたバージョンは当然誰も使ってないだろうという想定。