How to reproduce:
1. Open tester's project (CompileTestProject.zip)
2. Open "Test.unity" scene

Expected result: Code in "Test.cs" file compiles without problems
Actual result: Code in "Test.cs" file won't compile and throw errors at line 15

Reproducible with: 2018.1.0b1, 2018.1.0b12, 2018.2.0a7
Not reproducible with: 5.6.5.p3, 2017.1.3p2, 2017.2.2p2, 2017.3.2f1, 2017.4.1f1, 2018.1.0a1, 2018.1.0a7

Notes:

• Can be reproduced with both Mono versions

Upstream Bug: https://github.com/mono/mono/issues/7502

workaround is to store in a temp variable
change
(x * y).ToString();
to
var tmp = (x * y);
tmp.ToString();

Resolution: This is fixed at 2018.3 by using Roslyn rather than mcs when targeting the new scripting runtime.
Verified in: 2018.3.0a2