Re: [PATCH] c++: copy elision of object arg in static memfn call [PR110441]

[Jason Merrill via Gcc-patches]

On 7/13/23 14:49, Patrick Palka wrote:

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

OK.

-- >8 --

Here the call A().f() is represented as a COMPOUND_EXPR whose first
operand is the otherwise unused object argument A() and second operand
is the call result (both are TARGET_EXPRs).  Within the return statement,
this outermost COMPOUND_EXPR ends up foiling the copy elision check in
build_special_member_call, resulting in us introducing a bogus call to the
deleted move constructor.  (Within the variable initialization, which goes
through ocp_convert instead of convert_for_initialization, we've already
been eliding the copy despite the outermost COMPOUND_EXPR ever since
r10-7410-g72809d6fe8e085 made ocp_convert look through COMPOUND_EXPR).

In contrast, I noticed '(A(), A::f())' (which should be equivalent to
the above call) is represented with the COMPOUND_EXPR inside the RHS's
TARGET_EXPR initializer thanks to a special case in cp_build_compound_expr
thus avoiding the issue.

So this patch fixes this by making keep_unused_object_arg
use cp_build_compound_expr as well.

        PR c++/110441

gcc/cp/ChangeLog:

        * call.cc (keep_unused_object_arg): Use cp_build_compound_expr
        instead of building a COMPOUND_EXPR directly.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1z/elide8.C: New test.
---
  gcc/cp/call.cc                      |  2 +-
  gcc/testsuite/g++.dg/cpp1z/elide8.C | 25 +++++++++++++++++++++++++
  2 files changed, 26 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1z/elide8.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 119063979fa..b0a69cb46d4 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -5218,7 +5218,7 @@ keep_unused_object_arg (tree result, tree obj, tree fn)
    if (TREE_THIS_VOLATILE (a))
      a = build_this (a);
    if (TREE_SIDE_EFFECTS (a))
-    return build2 (COMPOUND_EXPR, TREE_TYPE (result), a, result);
+    return cp_build_compound_expr (a, result, tf_warning_or_error);
    return result;
  }

diff --git a/gcc/testsuite/g++.dg/cpp1z/elide8.C b/gcc/testsuite/g++.dg/cpp1z/elide8.C

new file mode 100644
index 00000000000..7d471be8a2a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/elide8.C
@@ -0,0 +1,25 @@
+// PR c++/110441
+// { dg-do compile { target c++11 } }
+
+struct immovable {
+  immovable(immovable &&) = delete;
+};
+
+struct A {
+  static immovable f();
+};
+
+immovable f() {
+  immovable m = A().f(); // { dg-error "deleted" "" { target c++14_down } }
+  return A().f(); // { dg-error "deleted" "" { target c++14_down } }
+}
+
+struct B {
+  A* operator->();
+};
+
+immovable g() {
+  B b;
+  immovable m = b->f(); // { dg-error "deleted" "" { target c++14_down } }
+  return b->f(); // { dg-error "deleted" "" { target c++14_down } }
+}