[gcc r12-3997] c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:9845c52db38f15740861435f38f7e5ad8a8de2ec

commit r12-3997-g9845c52db38f15740861435f38f7e5ad8a8de2ec
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 30 17:34:23 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.

Diff:
---
 gcc/cp/method.c                                        |  4 +++-
 gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C | 17 +++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 3c3495227ce..c38912a7ce9 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -2094,8 +2094,10 @@ 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..f6fbf8f2d9e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C
@@ -0,0 +1,17 @@
+// PR c++/102535
+// Verify __is_trivially_constructible works with multi-arg paren init of
+// aggrs.
+
+struct A { int x; };
+struct B { float y; };
+struct C { char z; };
+struct D { A a; B b; C c; };
+
+extern int n[1 + __is_trivially_constructible(D, A)];
+extern int n[1 + __is_trivially_constructible(D, A, B)];
+extern int n[1 + __is_trivially_constructible(D, A, B, C)];
+#if __cpp_aggregate_paren_init
+extern int n[1 + true];
+#else
+extern int n[1 + false];
+#endif