Unable to use extract_objects() for Vala unit tests

The following set up generates the error:
ninja: error: 'dummy@sha/meson-generated_Test.c.o', needed by 'passing_test', missing and no known rule to make it
When generated without the test the name is build/dummy@sha/meson-generated_src_Test.c.o. The _src has been dropped from the test rule.

meson.build:

project ('vala-unit-test', 'vala', 'c' )

deps = [
    dependency ('glib-2.0'),
    dependency ('gobject-2.0'),
]

sources = [
    'src/Test.vala',
]

lib = library ('dummy', sources: sources, dependencies: deps)

obj = lib.extract_objects ('src/Test.vala')
test = executable ('passing_test', sources: ['tests/TestShouldPass.vala'], dependencies: deps, objects: obj)
test('passing_test', test)

src/Test.vala:

class Test {
        public string value = "Test";
}

tests/TestShouldPass.vala:

void main () {
    var a = new Test ();
    assert (a.value == "Test");
}

Start with the minimal project in meson.build and reproduce the ninja error using src/Test.vala, tests/TestShouldPass.vala, and extract_objects(). Compare the generated object path with and without the test; done means the test target resolves the generated object and builds successfully.