Re: [PATCH 1/2] c++: Tolerate empty initial targs during normalization [PR97412]

[Jason Merrill via Gcc-patches]

On 10/29/20 11:21 AM, Patrick Palka wrote:

On Mon, 19 Oct 2020, Patrick Palka wrote:

When normalizing the constraint-expression of a nested-requirement, we
pass NULL_TREE as the initial template arguments for normalization, but
tsubst_argument_pack is not prepared to handle a NULL_TREE targ vector.
This causes us to ICE when normalizing a variadic concept as part of a
nested-requirement.

This patch fixes the ICE by guarding the call to tsubst_template_args in
normalize_concept_check appropriately.  This will also enables us to
simplify many of the normalization routines to pass NULL_TREE instead of
a set of generic template arguments as the initial template arguments,
which will be done in a subsequent patch.

Ping.  For some reason I confusingly referred to 'targs' in the commit
message when the variable in question is actually 'args'.  So I've
adjusted the commit message below:

OK.

-- >8 --

Subject: [PATCH] c++: Tolerate empty initial args during normalization
  [PR97412]

When normalizing the constraint-expression of a nested-requirement, we
pass NULL_TREE as the initial template arguments for normalization, but
tsubst_argument_pack is not prepared to handle a NULL_TREE args vector.
This causes us to ICE when normalizing a variadic concept as part of a
nested-requirement.

This patch fixes the ICE by guarding the call to tsubst_template_args in
normalize_concept_check appropriately.  This will also enable us to
simplify many of the normalization routines to just pass NULL_TREE
(instead of a set of generic template arguments) as the initial template
arguments.

gcc/cp/ChangeLog:

        PR c++/97412
        * constraint.cc (normalize_concept_check): Don't call
        tsubst_template_args when 'args' is NULL.

gcc/testsuite/ChangeLog:

        PR c++/97412
        * g++.dg/cpp2a/concepts-variadic2.C: New test.
---
  gcc/cp/constraint.cc                            |  3 ++-
  gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C | 12 ++++++++++++
  2 files changed, 14 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index f4f5174eff3..75457a2dd60 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -686,7 +686,8 @@ normalize_concept_check (tree check, tree args, norm_info 
info)
      }

/* Substitute through the arguments of the concept check. */

-  targs = tsubst_template_args (targs, args, info.complain, info.in_decl);
+  if (args)
+    targs = tsubst_template_args (targs, args, info.complain, info.in_decl);
    if (targs == error_mark_node)
      return error_mark_node;

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C

new file mode 100644
index 00000000000..ce61aef5481
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C
@@ -0,0 +1,12 @@
+// PR c++/97412
+// { dg-do compile { target c++20 } }
+
+template <class T, class... TArgs>
+concept call_bar_with = requires(T t, TArgs... args) {
+  t.bar(args...);
+};
+
+template <class T, class... TArgs>
+concept foo = requires {
+  requires call_bar_with<T, TArgs...>;
+};