I tested c++17 code on 4.27 and 5.0.1.
When I Built the project, 4.27 errored.
5.0.1 didn’t error and was a successful build(both full source and non full source projects).
I also tested c++20 code and both failed.
If you’re using the VSCode extension below you can force the Intellisense cpp standard to c++17, in the extension’s settings, when working on a UE5 project.
I believe there is a setting to make 4.27 use c++17 but with UE5 it’s not needed.