After installing Visual Studio 2015 and building my project I receive the error
"CSC : error CS7038: Failed to emit module".
Got the same error (fresh installation of the VS2015 Enterprise, ASP.NET webforms project .NET 4.0).
After some investigation I've found that there are two DLLs in references which causes this. Both are .Net 2.0 assemblies and both of them obfuscated by Red Gate Smart Assembly 6.5. And the real reason is... obfuscation.
Luckily, these assemblies are mine, so I've tried to build them without using of Smart Assembly - error is gone.
Interesting is that no any errors or warnings shown by Visual Studio before trying to build a project.
EDIT: Updating Smart Assembly to version 6.9 fixed an issue.