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The following code snippet causes a compiler warning:

interface Test {
  public abstract string test { get; set; }
}

class TestImpl : Test {
  public string test { get; set; }
}

void main() {
  new TestImpl() { test = "Test" };
}

early_init.vala.c:376:17: warning: passing argument 1 of ‘test_set_test’ from incompatible pointer type [-Wincompatible-pointer-types]
  376 |  test_set_test (_tmp1_, _tmp0_);
      |                 ^~~~~~
      |                 |
      |                 TestImpl * {aka struct _TestImpl *}
early_init.vala.c:92:22: note: expected ‘Test *’ {aka ‘struct _Test *’} but argument is of type ‘TestImpl *’ {aka ‘struct _TestImpl *’}
   92 | test_set_test (Test* self,

This is because the function is compiled to:

void
_vala_main (void)
{
        gchar* _tmp0_;
        TestImpl* _tmp1_ = NULL;
        TestImpl* _tmp2_;
        _tmp0_ = g_strdup ("Test");
        _tmp1_ = test_impl_new ();
        test_set_test (_tmp1_, _tmp0_);
        _g_free0 (_tmp0_);
        _tmp2_ = _tmp1_;
        _test_impl_unref0 (_tmp2_);
}

When changing main to

void main() {
  var test = new TestImpl();
  test.test = "Test";
}

the code is compiled to

void
_vala_main (void)
{
        TestImpl* test = NULL;
        TestImpl* _tmp0_;
        _tmp0_ = test_impl_new ();
        test = _tmp0_;
        test_set_test ((Test*) test, "Test");
        _test_impl_unref0 (test);
}

instead, which doesn't impose any warning (and also looks way cleaner in C, but less nice in Vala).