(Explicitly) Templated lambdas have a different signature to
implicitly templated lambdas -- '[]<template T> (T) {}' is not the
same as '[](auto) {}'.  This should be reflected in the mangling.  The
ABI captures this as
https://github.com/itanium-cxx-abi/cxx-abi/issues/31, and clang has
implemented such additions.

It's relatively straight forwards to write out the non-synthetic
template parms, and note if we need to issue an ABI warning.

I did find a couple of bugs on the way -- one is a failure to parse thhe pack 
expansion in :
   inline auto l_var2 = []<int... I> (int (&...)[I]) {}

the other was clang miscounting substitutions, 
https://github.com/llvm/llvm-project/issues/58631, always a good idea to have 
multiple implementations :)

the remaining change to do is lambda sequence numbering, as that is affected by 
lambda signature.

nathan

-- 
Nathan Sidwell