That is effectively a limitation inherited by the CLR. The default value must be encoded into the metadata of the parameter at compile time and available to allow any compiler to use that value in place of omitted arguments at the call site. There is no way to encode an expression that would be understood by the CLR or universally understood by CLR languages.

The argument could be made that for private or internal methods that the C# compiler could relax that restriction since the optional parameter metadata would never be a part of the public surface of the assembly. In those cases the compiler could contain a dictionary of the expressions and then replace the omitted arguments.

Something similar is already requested (in a way): #263 (closed, but could be re-opened if requirements are met), #2401.

Is it mostly about user-defined types or constants in general?

PS: please spend a 5 minutes to format the question better.

We are now taking language feature discussion on https://github.com/dotnet/csharplang for C# specific issues, https://github.com/dotnet/vblang for VB-specific features, and https://github.com/dotnet/csharplang for features that affect both languages.

Part of the proposal dotnet/csharplang#39 would address the use case for this issue.