Re: [PATCH] c++: deducing empty type vs non-type argument pack

[Jason Merrill via Gcc-patches]

On 7/18/23 11:18, Patrick Palka wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?  Also verified by way of gcc_assert that we never see
TEMPLATE_PARM_INDEX here.

OK.

-- >8 --

Within a template parameter list, a non-type template parameter pack is
represented as a PARM_DECL.  But in a couple of spots where we need to
deduce an empty argument pack, we test for TEMPLATE_PARM_INDEX (within a
template parameter list) instead of for PARM_DECL, which means we end up
using TYPE_ARGUMENT_PACK even in the non-type case.  This patch fixes
this (seemingly harmless) bug.

gcc/cp/ChangeLog:

        * pt.cc (type_unification_real): Test for PARM_DECL instead
        of TEMPLATE_PARM_INDEX to distinguish a type vs non-type
        template parameter pack.
        (type_targs_deducible_from): Likewise.
---
  gcc/cp/pt.cc | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 3987ffc509a..d342ab5929a 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23345,7 +23345,7 @@ type_unification_real (tree tparms,
            {
              tree arg;

- if (TREE_CODE (tparm) == TEMPLATE_PARM_INDEX)

+             if (TREE_CODE (tparm) == PARM_DECL)
                {
                  arg = make_node (NONTYPE_ARGUMENT_PACK);
                  TREE_CONSTANT (arg) = 1;
@@ -30369,7 +30369,7 @@ type_targs_deducible_from (tree tmpl, tree type)
        if (template_parameter_pack_p (tparm))
          {
            tree arg;
-           if (TREE_CODE (tparm) == TEMPLATE_PARM_INDEX)
+           if (TREE_CODE (tparm) == PARM_DECL)
              {
                arg = make_node (NONTYPE_ARGUMENT_PACK);
                TREE_CONSTANT (arg) = 1;