Because of the convertor function we have no guarantee that what we're
getting is in fact a `Dict[str, TYPE_var]`, it might well be anything in
the values, so we need to do some casting and set the return type to
object. This works out fine in practice as our declared `TypeDict`
inputs in the actual function signatures will be used instead.
This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.
As a side-effect from #8885 `find_program()` returns now `Executable`
objects when `meson.override_find_program` is called with an
executable target. To resolve this conflict the missing methods
from `ExternalProgram` are added to `BuildTarget`.
This allows checking specific values that are added or deprecated, which
we do a surprising amount of. This works with both containers and scalar
values
Daniel Mensinger
Dylan Baker
Xavier Claessens
Eli Schwartz