[Bug c++/82195] Undemangleable lambda

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82195

--- Comment #2 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
Created attachment 42171
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42171&action=edit
Further simplified

Further simplification removes the inner lambda.  The key problem is needing to
name objects within the scope of the instantiated generic lambda's template
function operator.  We end up using a template parameter reference, trying to
refer to that operator instantiation, but the scopes are confusing.

The Local::fn function is mangled as:
 _ZZZ3FoovENKUlT_E_clIiEEfS_EN5Local2fnEv
the backreference 'S_' is to the 'T_' component, but manually undoing that
substitution does not resolve the demangler's problem.

If we desugar the testcase, and define an explict lambda-like class, at file
scope, it's Local::fn function is mangled as:
  _ZZN6lambdaclIiEEfT_EN5Local2fnEv
The demangler can demangle that.  Manually wrapping that inside another
local-scope mangle breaks the demangler.