This patch implements Jason's suggestion of pushing a lambda scope when 
parsing a global variable initializer.  That bit worked fine, but 
happened to cause g++.dg/opt/dump1.C to not give any 
used-but-not-defined warnings.

The reason was no_linkage_check, which considers any lambda that has an 
extra-scope to have linkage.  Which is technically correct.  Except that 
we think that all types that have linkage have external linkage.

Our representation of linkage and visibility is somewhat inaccurate, 
particularly when it comes to types.  We have TREE_PUBLIC, 
DECL_EXTERNAL, DECL_VISIBILITY, DECL_COMDAT, DECL_NOT_REALLY_EXTERN.  It 
could really do with a through cleanup, but that won't be a simple task.

The best I could come up with was seeing if the extra scope was a 
VAR_DECL, and if that was TREE_PUBLIC and the var was inline (its 
COMDATness is sadly not set at that point) or a template instantiation, 
then the lambda had linkage.  Otherwise it's as-if it has no-linkage 
from the POV of compiler internals.

This is an ABI change (so we should document it), but it's changing 
mangling from an unpredictable (in practice) counter, to something the 
ABI defines.  So I'm not concerned about mangling-changed warnings, or 
preserving the broken mangling under some ABI selection flag.  Code that 
did this worked by accident within a single TU.  It'll continue to work 
by design there, and across TUs.

booted on x86_64-linux, I'll commit in a few days if there are no comments.

nathan

-- 
Nathan Sidwell