Using .NET 3.5, the C# project that is generated by Unity has a supported language version of 4:
<LangVersion>4</LangVersion>
However, Unity successfully compiles the project with C# 6 features like read-only auto properties, even though it passes `-langversion:4` to the compiler.

To reproduce:

1. Download attached project "LangVersionError.zip" and open in Unity
2. Open "CSharpScript.cs" in IDE by double-clicking file in Unity
3. Observe that IDE will report an Error similar to:

Feature 'readonly automatically implemented properties' is not available in C# 4. Please use language version 6 or greater.

Notes:
- This issue reproduces on both Windows and OSX
- In Unity everything is fine
- Discussion with #devs-scripting is in comments section

Reproduced on Unity 2017.1.3p4, 2017.2.3f1, 2017.3.2f1, 2017.4.2f2, 2018.1.0f2, 2018.2.0b3 and 2018.3.0a1