VAR and FIELD decls can become part of a lambda context, when the
lambda is 'attached' to that entity (It's a C++20 ODR thing that was
discovered with modules, but is actually separate.)  We were not
marking those decls as substitution candidates, leading to demangling
failures and compiler variance.

The alternative I chose is to bump the ABI, and add them to the
substitution table.  I think this is the intent of the ABI.

The other alternative would be for the demangler to pop off the most
recent substitution when it saw the 'M'.  But we'd have to change [at
least] the libiberty demangler[*] and the clang demangler and the clang
mangler, which seems a bigger challenge.

I'll commit this next week, unless y'all have comments.

We have other divergence from clang, with templated lambdas.  Clang 
added new manglings [Ty, Tn, Tt] for the lambda's template head (Richard 
has a pull request, but it's not merged 
https://github.com/itanium-cxx-abi/cxx-abi/issues/31).  Without that, we 
consider all the template args to be auto when demangling (which is 
going to look odd for non-type template args).  This divergence in 
signature also affects the lambda numbering, as gcc & clang differ in 
their opinions about 'same lamda signature'.   thoughts on addressing 
that to in this cycle?

I have some demangler fixes coming up.

nathan

[*] the demangler already seems to have a bug with 'M' handling.

-- 
Nathan Sidwell