Steps to reproduce:

1. Create new Unity project
2. Create a new script
3. Make sure that the "Scripting Runtime Version" is set to ".NET 4.x Equivalent" and "Api Compatibility Level" is set to ".NET Standard 2.0" (those values should be the default on latest versions)
4. Double-click the script to open it in Visual Studio
5. Notice that there's a bunch of "Predefined type 'System.Void' is not defined or imported." & "Predefined type 'System.Object' is not defined or imported." errors

Visual Studio for Mac 7.6.3 (build 1) used.

Reproduced with: 2018.1.3f1, 2018.2.7f1, 2018.3.0a1, 2018.3.0a11

Not reproduced with: 2018.1.2f1

Note: the default values of these settings are different in older versions so the error might not be seen right away even if they are reproducible.