Crash when calling LV2's extension_data()

[x42]

#0  0x00007ffff693246d in lilv_instance_get_extension_data (instance=0x18cb5890, uri=0x7ffff6a8e788 "http://harrisonconsoles.com/lv2/inlinedisplay#interface")
---Type <return> to continue, or q <return> to quit---
    at /home/ardour/linux-x86_64-v5/gtk/inst/include/lilv-0/lilv/lilv.h:1719
#1  0x00007ffff6938306 in ARDOUR::LV2Plugin::extension_data (this=0x18c9aca0, uri=0x7ffff6a8e788 "http://harrisonconsoles.com/lv2/inlinedisplay#interface")
    at ../libs/ardour/lv2_plugin.cc:1217
#2  0x00007ffff6934a05 in ARDOUR::LV2Plugin::init (this=0x18c9aca0, c_plugin=0x7e03620, rate=48000) at ../libs/ardour/lv2_plugin.cc:547
#3  0x00007ffff6933802 in ARDOUR::LV2Plugin::LV2Plugin (this=0x18c9aca0, engine=..., session=..., c_plugin=0x7e03620, rate=48000) at ../libs/ardour/lv2_plugin.cc:344
#4  0x00007ffff6943aa2 in ARDOUR::LV2PluginInfo::load (this=0x9558220, session=...) at ../libs/ardour/lv2_plugin.cc:3297
#5  0x00007ffff66d5bb3 in ARDOUR::find_plugin (session=..., identifier="http://portalmod.com/plugins/caps/AutoFilter", type=ARDOUR::LV2)
    at ../libs/ardour/plugin.cc:224
#6  0x00007ffff66eb5fc in ARDOUR::PluginInsert::set_state (this=0x18c90be0, node=..., version=3002) at ../libs/ardour/plugin_insert.cc:2490
#7  0x00007ffff677f2e4 in ARDOUR::Route::set_processor_state (this=0x18ba8ea0, node=...) at ../libs/ardour/route.cc:2884
#8  0x00007ffff677c426 in ARDOUR::Route::set_state (this=0x18ba8ea0, node=..., version=3002) at ../libs/ardour/route.cc:2490
#9  0x00007ffff691ba95 in ARDOUR::Track::set_state (this=0x18ba8ea0, node=..., version=3002) at ../libs/ardour/track.cc:125
#10 0x00007ffff66651c9 in ARDOUR::MidiTrack::set_state (this=0x18ba8ea0, node=..., version=3002) at ../libs/ardour/midi_track.cc:177
#11 0x00007ffff686f120 in ARDOUR::Session::XMLRouteFactory (this=0x305bd80, node=..., version=3002) at ../libs/ardour/session_state.cc:1745
#12 0x00007ffff686ea61 in ARDOUR::Session::load_routes (this=0x305bd80, node=..., version=3002) at ../libs/ardour/session_state.cc:1693
#13 0x00007ffff686db1d in ARDOUR::Session::set_state (this=0x305bd80, node=..., version=3002) at ../libs/ardour/session_state.cc:1593
#14 0x00007ffff6864167 in ARDOUR::Session::post_engine_init (this=0x305bd80) at ../libs/ardour/session_state.cc:287
#15 0x00007ffff67c0af6 in ARDOUR::Session::Session (this=0x305bd80, eng=..., fullpath="/media/hd/audio/cramned", snapshot_name="cramned", bus_profile=0x0, 
    mix_template="") at ../libs/ardour/session.cc:415
#16 0x0000000000734d7e in ARDOUR_UI::load_session (this=0x1b78680, path="/media/hd/audio/cramned", snap_name="cramned", mix_template="")
    at ../gtk2_ardour/ardour_ui.cc:3637
#17 0x000000000073450a in ARDOUR_UI::get_session_parameters (this=0x1b78680, quit_on_cancel=false, should_be_new=false, load_template="")
    at ../gtk2_ardour/ardour_ui.cc:3564
#18 0x0000000000729703 in ARDOUR_UI::starting (this=0x1b78680) at ../gtk2_ardour/ardour_ui.cc:1268
#19 0x00007ffff74f18d2 in Gtkmm2ext::UI::run (this=0x1b78680, old_receiver=...) at ../libs/gtkmm2ext/gtk_ui.cc:280
#20 0x0000000000c5c73c in main (argc=1, argv=0x7fffffffcd18) at ../gtk2_ardour/main.cc:413

(gdb) print _plugin_uri
$1 = 0x9562e50 "http://portalmod.com/plugins/caps/AutoFilter"

best guess: it's due to plugins/*/interface.cc returning a pointer to the Descriptor instance without static_cast<> to a LV2_Descriptor struct (and also no extern "C").

static const Descriptor<AutoFilter> lv2_descriptor(CAPS_URI "AutoFilter");
return i == 0 ? &lv2_descriptor : 0;

where Descriptor is-a DescriptorStub which is defined as

class DescriptorStub : public LADSPA_Descriptor, public LV2_Descriptor

The plugin host casts that pointer to a LV2_Descriptor without knowing about the Descriptor.