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

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Sun, Mar 6, 2016 at 1:42 AM, Jason Merrill <jason@redhat.com> wrote:
> 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.

Sorry about that.

In the case of LAMBDA_TYPE_EXTRA_SCOPE being a PARM_DECL, could there
be a chance that this PARM_DECL has a non-null DECL_LANG_SPECIFIC? If
so it looks like we could still later ICE in get_template_info (called
from template_class_depth) when we try to get at its
DECL_TEMPLATE_INFO, an accessor that cannot be used on PARM_DECLs.  So
I wonder if maybe get_template_info() should also be adjusted to
handle the case of being given a PARM_DECL which may have a non-null
DECL_LANG_SPECIFIC:

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 823e2f0..f68b1f6 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -330,7 +330,8 @@ get_template_info (const_tree t)
   if (!t || t == error_mark_node)
     return NULL;

-  if (TREE_CODE (t) == NAMESPACE_DECL)
+  if (TREE_CODE (t) == NAMESPACE_DECL
+      || TREE_CODE (t) == PARM_DECL)
     return NULL;

   if (DECL_P (t) && DECL_LANG_SPECIFIC (t))
@@ -378,7 +379,7 @@ template_class_depth (tree type)
          && uses_template_parms (INNERMOST_TEMPLATE_ARGS (TI_ARGS (tinfo))))
        ++depth;

-      if (VAR_OR_FUNCTION_DECL_P (type))
+      if (DECL_P (type))
        type = CP_DECL_CONTEXT (type);
       else if (LAMBDA_TYPE_P (type))
        type = LAMBDA_TYPE_EXTRA_SCOPE (type);