Re: [PATCH] c++: variadic class placeholder template deduction [PR97134]

[Jason Merrill via Gcc-patches]

On 4/9/21 5:25 PM, Patrick Palka wrote:

do_class_deduction handles specially the case where we're deducing one
placeholder from another equivalent one, but here the initializer passed
to do_class_deduction is wrapped in an EXPR_PACK_EXPANSION (we're being
called from unify during get_partial_spec_bindings).  This patch makes
do_class_deduction look through EXPR_PACK_EXPANSIONs so that we detect
this case as well.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk or perhaps stage1?

OK for trunk.

gcc/cp/ChangeLog:

        PR c++/97134
        * pt.c (do_class_deduction): Look through EXPR_PACK_EXPANSION
        when checking if the initializer is an equivalent class
        placeholder template parameter.

gcc/testsuite/ChangeLog:

        PR c++/97134
        * g++.dg/cpp2a/nontype-class43.C: New test.
---
  gcc/cp/pt.c                                  |  6 +++++-
  gcc/testsuite/g++.dg/cpp2a/nontype-class43.C | 11 +++++++++++
  2 files changed, 16 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class43.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 01c807be1bb..872b08ec9ad 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29265,7 +29265,11 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
      return ptype;

/* Initializing one placeholder from another. */

-  if (init && TREE_CODE (init) == TEMPLATE_PARM_INDEX
+  if (init
+      && (TREE_CODE (init) == TEMPLATE_PARM_INDEX
+         || (TREE_CODE (init) == EXPR_PACK_EXPANSION
+             && (TREE_CODE (PACK_EXPANSION_PATTERN (init))
+                 == TEMPLATE_PARM_INDEX)))
        && is_auto (TREE_TYPE (init))
        && CLASS_PLACEHOLDER_TEMPLATE (TREE_TYPE (init)) == tmpl)
      return cp_build_qualified_type (TREE_TYPE (init), cp_type_quals (ptype));
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class43.C 
b/gcc/testsuite/g++.dg/cpp2a/nontype-class43.C
new file mode 100644
index 00000000000..f51684f7a62
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class43.C
@@ -0,0 +1,11 @@
+// PR c++/97134
+// { dg-do compile { target c++20 } }
+
+template<typename T>
+struct templ {};
+
+template<templ... Vs>
+struct wrapper {};
+
+template<templ... Vs> requires true
+struct wrapper<Vs...> {};