Thanks for your pull request, @wilzbach!

Bugzilla references

We should keep things orthogonal: where we output json (file vs stdout) and what (with or without input files):

• -Xf=outputfile should also print all static info to outputfile if no input files are given (same as -X)

• outputting to stdout (eg via -X or whatever syntax is used) should behave exactly as -Xf=outputfile (except outputting to stdout) even when input files are given

• There's a change of behavior involved: -X currently outputs json to mainfile.json;

• Why not make things simple and use -Xf=- to mean stdout? following a common convention: https://unix.stackexchange.com/questions/16357/usage-of-dash-in-place-of-a-filename

There's a change of behavior involved: -X currently outputs json to mainfile.json;

No. dmd -X will just print the help page, but if -Xf=- makes everyone happy, I don't mind to change it do this.

-Xf=outputfile should also print all static info to outputfile if no input files are given (same as-X)

Ok. I will change to this soon.

what i meant was:
dmd -X main.d currently writes main.json so this PR introduces a change of behavior if -X were to mean stdout; hence the new syntax -Xf=- is nice because it doesn't introduce a change of behavior, and is also intuitive and standard

FYI #7838 comes with the same behavior and actually already does what this PR was trying to do, so let's better move the discussion to that PR.