Re: [PATCH] Fix PR c++/66786 (ICE with nested lambdas in variable template)

[Jason Merrill <jason at redhat dot com>]

On 02/08/2016 12:19 AM, Patrick Palka wrote:
> Here, we are calling template_class_depth on a FIELD_DECL corresponding
> to a lambda that is used inside variable template.  template_class_depth
> however does not see that this FIELD_DECL is used inside a variable
> template binding because its chain of DECL_CONTEXTs does not include the
> corresponding VAR_DECL.  So template_class_depth returns the wrong
> template nesting level which causes its callers to malfunction.  In
> particular we strip a template argument level in
> tsubst_copy [FIELD_DECL] when we shouldn't have.
>
> This patch makes template_class_depth look at a lambda type's
> LAMBDA_TYPE_EXTRA_SCOPE field instead of its TYPE_CONTEXT, so that it
> can iterate into an enclosing variable template, if applicable.
>
> Tested on x86_64-pc-linux gnu, no new regressions.  Also tested against
> Boost.  Is this OK to commit?

This is breaking several lambda testcases with -fconcepts on; 
LAMBDA_TYPE_EXTRA_SCOPE can also be a FIELD_DECL or PARM_DECL.  Let's 
just check DECL_P.

Jason