* [pushed] c++: Avoid calling const copy ctor on implicit move. [PR91212]
@ 2020-07-29 18:24 Jason Merrill
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From: Jason Merrill @ 2020-07-29 18:24 UTC (permalink / raw)
To: gcc-patches
Our implementation of C++11 implicit move was wrong for return; we didn't
actually hit the check for the type of the first parameter of the selected
constructor, because we didn't see LOOKUP_PREFER_RVALUE set properly.
Fixing that to look at the right flags fixed the issue for this testcase,
but broke implicit move for a by-value converting constructor (PR58051). I
think this was not allowed in C++17, but it is allowed under the implicit
move changes from C++20, and those changes were voted to apply as a DR to
earlier standards as well, so I don't want to break it now.
So after fixing the flags check I changed the test to allow value
parameters.
Tested x86_64-pc-linux-gnu, applying to trunk.
gcc/cp/ChangeLog:
PR c++/91212
* call.c (build_over_call): Don't call a const ref
overload for implicit move.
gcc/testsuite/ChangeLog:
PR c++/91212
* g++.dg/cpp0x/move-return3.C: New test.
---
gcc/cp/call.c | 9 ++++++---
gcc/testsuite/g++.dg/cpp0x/move-return3.C | 23 +++++++++++++++++++++++
2 files changed, 29 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/move-return3.C
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index f9508ae3635..e283d635d60 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8678,15 +8678,18 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
parm = TREE_CHAIN (parm);
}
- if (flags & LOOKUP_PREFER_RVALUE)
+ if (cand->flags & LOOKUP_PREFER_RVALUE)
{
/* The implicit move specified in 15.8.3/3 fails "...if the type of
the first parameter of the selected constructor is not an rvalue
reference to the object's type (possibly cv-qualified)...." */
gcc_assert (!(complain & tf_error));
tree ptype = convs[0]->type;
- if (!TYPE_REF_P (ptype)
- || !TYPE_REF_IS_RVALUE (ptype)
+ /* Allow calling a by-value converting constructor even though it
+ isn't permitted by the above, because we've allowed it since GCC 5
+ (PR58051) and it's allowed in C++20. But don't call a copy
+ constructor. */
+ if ((TYPE_REF_P (ptype) && !TYPE_REF_IS_RVALUE (ptype))
|| CONVERSION_RANK (convs[0]) > cr_exact)
return error_mark_node;
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/move-return3.C b/gcc/testsuite/g++.dg/cpp0x/move-return3.C
new file mode 100644
index 00000000000..c79f0591936
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/move-return3.C
@@ -0,0 +1,23 @@
+// PR c++/91212
+// Test that C++11 implicit move semantics don't call the const copy.
+// { dg-do link }
+
+struct T { int i; };
+
+struct X {
+ X(T&) { } // #1
+ X(const T&); // #2
+};
+
+X
+fn ()
+{
+ T buf;
+ return buf;
+}
+
+int
+main()
+{
+ X c = fn ();
+}
base-commit: d8140b9ed3c0fed041aedaff3fa4a603984ca10f
--
2.18.1
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2020-07-29 18:24 [pushed] c++: Avoid calling const copy ctor on implicit move. [PR91212] Jason Merrill
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