meson/docs/markdown/snippets/fallback_bool.md

## Controlling subproject dependencies with  `dependency(allow_fallback: ...)`

As an alternative to the `fallback` keyword argument to `dependency`,
you may use `allow_fallback`, which accepts a boolean value. If `true`
and the dependency is not found on the system, Meson will fallback
to a subproject that provides this dependency, even if the dependency
is optional. If `false`, Meson will not fallback even if a subproject
provides this dependency.
dependency: support boolean argument "allow_fallback" Sometimes, distros want to configure a project so that it does not use any bundled library. In this case, meson.build might want to do something like this, where slirp is a combo option with values auto/system/internal: slirp = dependency('', required: false) if get_option('slirp') != 'internal' slirp = dependency('slirp', required: get_option('slirp') == 'system') endif if not slirp.found() slirp = subproject('libslirp', ...) .variable('...') endif and we cannot use "fallback" because the "system" value should never look for a subproject. This worked until 0.54.x, but in 0.55.x this breaks because of the automatic subproject search. Note that the desired effect here is backwards compared to the policy of doing an automatic search on "required: true"; we only want to do the search if "required" is false! It would be possible to look for the dependency with `required: false` and issue the error manually, but it's ugly and it may produce an error message that looks "different" from Meson's. Instead, with this change it is possible to achieve this effect in an even simpler way: slirp = dependency('slirp', required: get_option('slirp') != 'auto', allow_fallback: get_option('slirp') == 'system' ? false : ['slirp', 'libslirp_dep']) The patch also adds support for "allow_fallback: true", which is simple and enables automatic fallback to a wrap even for non-required dependencies. 4 years ago			## Controlling subproject dependencies with `dependency(allow_fallback: ...)`

			As an alternative to the `fallback` keyword argument to `dependency`,
			you may use `allow_fallback`, which accepts a boolean value. If `true`
			`and the dependency is not found on the system, Meson will fallback`
			`to a subproject that provides this dependency, even if the dependency`
			is optional. If `false`, Meson will not fallback even if a subproject
			`provides this dependency.`