#! "shebang" directives have changed from disallowed to allowed in ordinary files in recent compilers

[RikkiGibson]

Repro:

• install .NET 10 preview SDK
• Shell:

$ dotnet new console -f net9.0 -o MyApp
$ cd MyApp
$ dotnet new globaljson

• Insert #!something at top of file
• dotnet build

Expected result: Program.cs(1,1): error CS1024: Preprocessor directive expected
Actual result: Build succeeded

If you edit the global.json to use .NET 9 SDK and build again, the expected error occurs.

In the ignored directives design we decided to block #: in the context of ordinary projects. It's not obvious to me if we decided to lift the pre-existing restriction on #! in ordinary projects. I expected the design to align, where in .cs files, either both #! and #: are permitted, or both are disallowed.

It's quite possible we discussed and accepted the behavior described in this bug, in which case please link it and close out.

[jjonescz]

That #: directives are limited to -features:FileBasedPrograms is a roslyn implementation detail, nevertheless the language proposal talks about it here:

Compilers are also free to report errors if these directives are used in unsupported scenarios, e.g., Roslyn will report an error if these directives are present in a file compiled as part of "project-based programs" as opposed to "file-based programs" (and tooling will remove these directives when migrating file-based programs to project-based programs). That error should not be reported for the #! directive, it can be placed on any file because it might invoke some other tool than dotnet run.

The last sentence says the limitation should not apply to #!.

I don't remember this detail being discussed at LDM, but it was in the proposal at the time when it was approved.

[jaredpar]

@jjonescz let's do a quick check in with LDM about this. Going to keep issue open to track that for now.