[gcc r11-9085] c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]

[gcc r11-9085] c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]

[Patrick Palka]

https://gcc.gnu.org/g:d80c0e4584acc204ca9a2f8880ad455fef48371f

commit r11-9085-gd80c0e4584acc204ca9a2f8880ad455fef48371f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 30 17:54:17 2021 -0400

    c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]
    
    is_xible_helper assumes only 0- and 1-argument ctors can be trivial, but
    C++20 aggregate paren init means multi-arg ctors can now be trivial too.
    This patch relaxes the relevant early exit check accordingly.
    
            PR c++/102535
    
    gcc/cp/ChangeLog:
    
            * method.c (is_xible_helper): Don't exit early for multi-arg
            ctors in C++20.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/is_trivially_constructible7.C: New test.
    
    (cherry picked from commit 9845c52db38f15740861435f38f7e5ad8a8de2ec)

Diff:
---
 gcc/cp/method.c                                    |  4 +++-
 .../g++.dg/ext/is_trivially_constructible7.C       | 23 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 8d2f3ce30af..0489e2f9282 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -2092,8 +2092,10 @@ is_xible_helper (enum tree_code code, tree to, tree from, bool trivial)
   tree expr;
   if (code == MODIFY_EXPR)
     expr = assignable_expr (to, from);
-  else if (trivial && from && TREE_CHAIN (from))
+  else if (trivial && from && TREE_CHAIN (from)
+	   && cxx_dialect < cxx20)
     return error_mark_node; // only 0- and 1-argument ctors can be trivial
+			    // before C++20 aggregate paren init
   else if (TREE_CODE (to) == ARRAY_TYPE && !TYPE_DOMAIN (to))
     return error_mark_node; // can't construct an array of unknown bound
   else
diff --git a/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C b/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C
new file mode 100644
index 00000000000..38531074be3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C
@@ -0,0 +1,23 @@
+// PR c++/102535
+// Verify __is_trivially_constructible works with multi-arg paren init of
+// aggrs.
+// { dg-do compile { target c++11 } }
+
+struct A { int x; };
+struct B { float y; };
+struct C { char z; };
+struct D { A a; B b; C c; };
+
+template<class T, class... Ts>
+struct is_trivially_constructible {
+  static const bool value = __is_trivially_constructible(T, Ts...);
+};
+
+extern int n[1 + is_trivially_constructible<D, A>::value];
+extern int n[1 + is_trivially_constructible<D, A, B>::value];
+extern int n[1 + is_trivially_constructible<D, A, B, C>::value];
+#if __cpp_aggregate_paren_init
+extern int n[1 + true];
+#else
+extern int n[1 + false];
+#endif