From 72bdfcb848327020f62f72405d72cf85650666e1 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Sat, 26 Mar 2022 20:38:54 -0400
Subject: [PATCH] c++: missing aggregate base ctor [PR102045]

When make_base_init_ok changes a call to a complete constructor into a call
to a base constructor, we were never marking the base ctor as used, so it
didn't get emitted.

	PR c++/102045

gcc/cp/ChangeLog:

	* call.cc (make_base_init_ok): Call make_used.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/aggr-base12.C: New test.
---
 gcc/cp/call.cc                           |  1 +
 gcc/testsuite/g++.dg/cpp1z/aggr-base12.C | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/aggr-base12.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index dfe370d685df..73fede5a3df4 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -8958,6 +8958,7 @@ make_base_init_ok (tree exp)
        call target.  It would be possible to splice in the appropriate
        arguments, but probably not worth the complexity.  */
     return false;
+  mark_used (fn);
   AGGR_INIT_EXPR_FN (exp) = build_address (fn);
   return true;
 }
diff --git a/gcc/testsuite/g++.dg/cpp1z/aggr-base12.C b/gcc/testsuite/g++.dg/cpp1z/aggr-base12.C
new file mode 100644
index 000000000000..6f5a6b2056af
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/aggr-base12.C
@@ -0,0 +1,24 @@
+// PR c++/102045
+// { dg-do link { target c++17 } }
+
+template<typename T>
+struct span
+{
+  template<unsigned long N>
+  constexpr span(T (&a)[N]) : data(a), len(N) { }
+  constexpr bool empty() const { return len == 0; }
+  T* data;
+  unsigned long len;
+};
+
+struct byte_writer: span<char> {
+  constexpr void do_something() noexcept {
+    (void)this->empty();
+  }
+};
+
+int main() {
+  char array[1];
+  auto writer = byte_writer{array};
+  writer.do_something();
+}
-- 
2.47.2