meson/docs/markdown/snippets/implicit_fallback.md

Implicit dependency fallback

dependency('foo') now automatically fallback if the dependency is not found on the system but a subproject wrap file or directory exists with the same name.

That means that simply adding subprojects/foo.wrap is enough to add fallback to any dependency('foo') call. It is however requires that the subproject call meson.override_dependency('foo', foo_dep) to specify which dependency object should be used for foo.

Wrap file provide section

Wrap files can define the dependencies it provides in the [provide] section. When a wrap file provides the dependency foo any call do dependency('foo') will automatically fallback to that subproject even if no fallback keyword argument is given. See Wrap documentation.

find_program() fallback

When a program cannot be found on the system but a wrap file has its name in the [provide] section, that subproject will be used as fallback.