问GME在unity 2019.1.8.f1中编译失败！？

unity 2019.1.8.f1中编译失败，多处报类似以下的错误：

Library/PackageCache/com.unity.textmeshpro@2.0.1/Scripts/Runtime/FastAction.cs(24,45): error CS1644: Feature `out variable declaration' cannot be used because it is not part of the C# 4.0 language specification