Library has odd behavior if a user defined version flag is specified

[ThadHouse]

Was converting an old app that had a --version flag to System.CommandLine. It seems that there's also a --version flag integrated into System.CommandLine, which was taking precedence. System.CommandLine should error if one of the specified root options interferes with one of the built in options.

[jonsequitur]

Was your custom option also added at the root? (If added to a subcommand, it should shadow the recursive options added higher up the command tree.)

This has always been a bit of a gotcha. The library used to validate collisions and throw but we decided it wasn't worth the runtime cost. A source generator or analyzer or other build-time mechanism was proposed but not built.

One approach that can work as a stopgap is to write a test for collisions. Here's an example:

https://github.com/dotnet/interactive/blob/35f24e5e4e1b858e9808ea10e6010501dcecb739/src/dotnet-interactive.Tests/CommandLine/CommandExtensions.cs#L16