Reproduction steps:

1. Open the attached project
2. Drag the “/Assets/PartialClass.cs” from the Project window onto any GameIbject in the Hierarchy window
3. Observe

Expected result: The Script is added as a Component to the GameObject
Actual result: “Can’t add script” error window pops up

Reproducible with: 2022.2.0a5, 2022.2.0b8, 2023.1.11f1
Not reproducible with: 2020.3.39f1, 2021.3.10f1, 2022.1.17f1, 2022.2.0a4

Reproducible on: M1 MacOS 12.4 (21F79), Windows 10 (21H2)

Notes:

• This happens when a single Script file contains 2 partial classes and one of them inherits from MonoBehavior, if you split these 2 partial classes into 2 separate Script files with one partial class each, it works as expected
• The issue is reproducible on both Intel and Apple Silicon Editors on M1 Mac