This is a very important clarification)) Thanks for editing the message. Is there an information somewhere about toolchains support? I mean in what IDEs they can be used and what are the limitations.
I've searched the forum but found not so much useful information: Open-source toolchains for Playgrounds (iOS, Xcode 8) - #3 by Karl
New project with single view application also fails to compile in SwiftUI preview and via build command.
I'm a bit confused as there are settings in XCode>Toolchains menu where Swift 6 toolchain can be selected which hints it should be supported.
Previously I've only used toolchains from different XCode versions and they worked as expected.