From: Jason Merrill <jason@redhat.com>
Date: Wed, 13 Jan 2021 18:27:06 +0000 (-0500)
Subject: c++: Avoid redundant copy in {} init [PR98642]
X-Git-Tag: releases/gcc-10.3.0~352
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=506fcfdb12562e829448e2b2eff475c65986e26b;p=thirdparty%2Fgcc.git

c++: Avoid redundant copy in {} init [PR98642]

Here, initializing from { } implies a call to the default constructor for
base.  We were then seeing that we're initializing a base subobject, so we
tried to copy the result of that call.  This is clearly wrong; we should
initialize the base directly from its default constructor.

gcc/cp/ChangeLog:

	PR c++/98642
	* typeck2.c (split_nonconstant_init_1): Don't copy a list-init
	constructor call.

gcc/testsuite/ChangeLog:

	PR c++/98642
	* g++.dg/cpp1z/elide5.C: New test.
---

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index c3b29cec1adb..2fc6081557f2 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -706,17 +706,31 @@ split_nonconstant_init_1 (tree dest, tree init, bool nested)
 		    sub = build3 (COMPONENT_REF, inner_type, dest, field_index,
 				  NULL_TREE);
 
+		  /* We may need to add a copy constructor call if
+		     the field has [[no_unique_address]].  */
 		  if (unsafe_return_slot_p (sub))
 		    {
-		      /* We may need to add a copy constructor call if
-			 the field has [[no_unique_address]].  */
+		      /* But not if the initializer is an implicit ctor call
+			 we just built in digest_init.  */
+		      if (TREE_CODE (value) == TARGET_EXPR
+			  && TARGET_EXPR_LIST_INIT_P (value))
+			{
+			  tree init = TARGET_EXPR_INITIAL (value);
+			  if (init && TREE_CODE (init) == AGGR_INIT_EXPR
+			      && AGGR_INIT_VIA_CTOR_P (init))
+			    goto build_init;
+			}
+
 		      releasing_vec args = make_tree_vector_single (value);
 		      code = build_special_member_call
 			(sub, complete_ctor_identifier, &args, inner_type,
 			 LOOKUP_NORMAL, tf_warning_or_error);
 		    }
 		  else
-		    code = build2 (INIT_EXPR, inner_type, sub, value);
+		    {
+		    build_init:
+		      code = build2 (INIT_EXPR, inner_type, sub, value);
+		    }
 		  code = build_stmt (input_location, EXPR_STMT, code);
 		  code = maybe_cleanup_point_expr_void (code);
 		  add_stmt (code);
diff --git a/gcc/testsuite/g++.dg/cpp1z/elide5.C b/gcc/testsuite/g++.dg/cpp1z/elide5.C
new file mode 100644
index 000000000000..abe80ec0bc43
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/elide5.C
@@ -0,0 +1,15 @@
+// PR c++/98642
+// { dg-do compile { target c++11 } }
+
+struct base {
+  base(void) {}
+  base(base &&) = delete;
+};
+
+struct foo : public base { };
+
+static foo c1 { };
+
+#if __cpp_aggregate_bases
+static foo c2 { {} };
+#endif