add instantiation of plugin list

[tjhei]

Clang complains

./aspect/include/aspect/plugins.h:731:13: error: instantiation of
variable
'aspect::internal::Plugins::PluginList<aspect::BoundaryTemperature::Interface<3>>::plugins'
required here, but no definition is available
[-Werror,-Wundefined-var-template]

[bangerth]

The line you add doesn't actually do anything. It just says "if you have an instantiation of the class, this member variable exists and is initialized as follows". But unless you have a unity build, you're not actually instantiating anything in this file.

I believe it should work if you moved the lines in question into the header file.

[tjhei]

I believe it should work if you moved the lines in question into the header file.

I am getting linker errors if I do that.

Here is what I am trying to fix by the way (maybe I need to run this locally instead of guessing blindly):
https://github.com/geodynamics/aspect/actions/runs/19410474400/job/55531108392#step:5:173

[bangerth]

Do you have an explicit instantiation of Plugins::PluginList<aspect::BoundaryTemperature::Interface<3>> anywhere (perhaps via a macro)? If so, that place also needs to explicitly instantiate Plugins::PluginList<aspect::BoundaryTemperature::Interface<3>>::plugins.