c++: Make braced-init-list as template arg work with aggr init [PR95369]

Barry pointed out to me that our braced-init-list as a template-argument
extension doesn't work as expected when we aggregate-initialize.  Since
aggregate list-initialization is a user-defined conversion sequence, we
allow it as part of a converted constant expression.

Co-authored-by: Jason Merrill <jason@redhat.com>

gcc/cp/ChangeLog:

	PR c++/95369
	* call.c (build_converted_constant_expr_internal): Allow
	list-initialization.

gcc/testsuite/ChangeLog:

	PR c++/95369
	* g++.dg/cpp2a/nontype-class38.C: New test.

2 files changed, 33 insertions, 1 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 2b393f9..3c97b98 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -4348,7 +4348,7 @@ build_converted_constant_expr_internal (tree type, tree expr,
      and where the reference binding (if any) binds directly.  */
 
   for (conversion *c = conv;
-       conv && c->kind != ck_identity;
+       c && c->kind != ck_identity;
        c = next_conversion (c))
     {
       switch (c->kind)
@@ -4356,6 +4356,8 @@ build_converted_constant_expr_internal (tree type, tree expr,
 	  /* A conversion function is OK.  If it isn't constexpr, we'll
 	     complain later that the argument isn't constant.  */
 	case ck_user:
+	  /* List-initialization is OK.  */
+	case ck_aggr:
 	  /* The lvalue-to-rvalue conversion is OK.  */
 	case ck_rvalue:
 	  /* Array-to-pointer and function-to-pointer.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
new file mode 100644
index 0000000..5b440fd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
@@ -0,0 +1,30 @@
+// PR c++/95369
+// { dg-do compile { target c++20 } }
+
+struct S {
+  int a;
+  int b;
+};
+
+struct W {
+  int i;
+  S s;
+};
+
+template <S p>
+void fnc()
+{
+}
+
+template<S s> struct X { };
+template<W w> struct Y { };
+
+void f()
+{
+  fnc<{ .a = 10, .b = 20 }>();
+  fnc<{ 10, 20 }>();
+  X<{ .a = 1, .b = 2 }> x;
+  X<{ 1, 2 }> x2;
+  // Brace elision is likely to be allowed.
+  Y<{ 1, 2, 3 }> x3;
+}