When checking constraint satisfaction on template arguments, the outer
template levels are not added to MultiLevelTemplateArgumentList.
So I don't think this is the right fix for this... But I DID check and see that the test case seems to be fixed here: https://reviews.llvm.org/D126907 , which does a lot of rework for this sort of thing.
That patch is expected to be complete/ready for submission in the 'near future', but I'm still working through 1 more issue.