There used to be a warning shown after trying to add a script just after creating it and while the engine was compiling all the scripts. There is no longer a warning, only an error:
"Can't add script behaviour VisualContainerAsset. The script needs to derive from MonoBehaviour"

To reproduce:
1. Open user's attached project or any other project with several scripts in it
2. Create a new script and name it
3. Right after naming it, while it's compiling, attach the script to any gameobject in the scene
4. Observe the error

Expected result: A warning occurs that script cannot be added while the compilation is in progress
Actual result: An error occurs informing that the script needs to derive from MonoBehaviour

Reproduced with: 2018.1.0a5, 2018.1.0b6
Not reproduced with: 2017.1.3p1, 2017.2.1p3, 2017.3.1f1, 2018.1.0a4
Regression started in: 2018.1.0a5

Fixed in 2018.2.0a4