Currently, building IDL targets requires invoking a bit of python code in the sources list to generate the c++ sources.  For example:

env.Library(
    target='some_library',
    source=[
        'foo.cpp',
        env.Idlc('bar.idl')[0],
    ],
)

The env.Idlc() invocation creates the generated c++ source and header files and SCons ultimately sees:

env.Library(
    target='some_library',
    source=[
        'foo.cpp',
        'bar_gen.cpp',
    ],
)

It would be a small, but repeated quality of life improvement to teach SCons about .idl files explicitly so that source lists could be written more simply and readably as:

env.Library(
    target='some_library',
    source=[
        'foo.cpp',
        'bar.idl',
    ],
)