If a package is defined in provide section:
and the same extension is installed in the system composer gives an error.
It would be nice to give priority to installed packages and ignore the ones in this section.
PS: I'm using this because I'm developing on widows where the extension is not installed but when I used my linux test server it's there.
There is now a --ignore-platform-reqs flag for install/update that allows you to install even if an ext is missing on windows, but #1522 would be a better fix for what you are doing I guess. Anyway I'm closing this as there is a workable solution but I'd suggest you track #1522 (should be fixed soon too).