The bug is confirmed to be fixed on Swift 5.8 with an error message "Initializer for conditional binding must have Optional type, not 'Int'" like the normal code.
Thanks you for the rewrite of ResultBuilder @xedin
See the post for more detail: Improved Result Builder Implementation in Swift 5.8 - #13 by xedin
And does it possible to add such code to the test case for ResultBuilder in case the bug regress on later Swift version?