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From: Jakub Jelinek <jakub@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc(refs/vendors/redhat/heads/gcc-8-branch)] PR c++/86521 - C++17 copy elision in initialization by constructor. Date: Thu, 17 Sep 2020 16:47:30 +0000 (GMT) [thread overview] Message-ID: <20200917164730.052AE3987925@sourceware.org> (raw) https://gcc.gnu.org/g:9e706302753b066b6e5765ecc32e0381c4a4581d commit 9e706302753b066b6e5765ecc32e0381c4a4581d Author: Jason Merrill <jason@redhat.com> Date: Wed Feb 26 00:33:52 2020 -0500 PR c++/86521 - C++17 copy elision in initialization by constructor. This is an overlooked case in C++17 mandatory copy elision: We want overload resolution to reflect that initializing an object from a prvalue does not involve a copy or move constructor even when [over.match.ctor] says that only constructors are candidates. Here I implement that by looking through the copy/move constructor in joust. gcc/cp/ChangeLog 2020-02-26 Jason Merrill <jason@redhat.com> PR c++/86521 - C++17 copy elision in initialization by constructor. * call.c (joust_maybe_elide_copy): New. (joust): Call it. Diff: --- gcc/cp/ChangeLog | 6 ++++ gcc/cp/call.c | 53 ++++++++++++++++++++++++++++ gcc/testsuite/g++.dg/cpp0x/overload-conv-3.C | 2 +- gcc/testsuite/g++.dg/overload/conv-op2.C | 6 ++-- 4 files changed, 63 insertions(+), 4 deletions(-) diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog index 1acd99f4a05..260f38f0b58 100644 --- a/gcc/cp/ChangeLog +++ b/gcc/cp/ChangeLog @@ -1,3 +1,9 @@ +2020-02-26 Jason Merrill <jason@redhat.com> + + PR c++/86521 - C++17 copy elision in initialization by constructor. + * call.c (joust_maybe_elide_copy): New. + (joust): Call it. + 2020-02-26 Jason Merrill <jason@redhat.com> PR c++/90546 diff --git a/gcc/cp/call.c b/gcc/cp/call.c index 2f575620b25..3b0b216d2ea 100644 --- a/gcc/cp/call.c +++ b/gcc/cp/call.c @@ -10057,6 +10057,33 @@ add_warning (struct z_candidate *winner, struct z_candidate *loser) winner->warnings = cw; } +/* CAND is a constructor candidate in joust in C++17 and up. If it copies a + prvalue returned from a conversion function, replace CAND with the candidate + for the conversion and return true. Otherwise, return false. */ + +static bool +joust_maybe_elide_copy (z_candidate *&cand) +{ + tree fn = cand->fn; + if (!DECL_COPY_CONSTRUCTOR_P (fn) && !DECL_MOVE_CONSTRUCTOR_P (fn)) + return false; + conversion *conv = cand->convs[0]; + gcc_checking_assert (conv->kind == ck_ref_bind); + conv = next_conversion (conv); + if (conv->kind == ck_user && !TYPE_REF_P (conv->type)) + { + gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p + (conv->type, DECL_CONTEXT (fn))); + z_candidate *uc = conv->cand; + if (DECL_CONV_FN_P (uc->fn)) + { + cand = uc; + return true; + } + } + return false; +} + /* Compare two candidates for overloading as described in [over.match.best]. Return values: @@ -10137,6 +10164,27 @@ joust (struct z_candidate *cand1, struct z_candidate *cand2, bool warn, } } + /* Handle C++17 copy elision in [over.match.ctor] (direct-init) context. The + standard currently says that only constructors are candidates, but if one + copies a prvalue returned by a conversion function we want to treat the + conversion as the candidate instead. + + Clang does something similar, as discussed at + http://lists.isocpp.org/core/2017/10/3166.php + http://lists.isocpp.org/core/2019/03/5721.php */ + int elided_tiebreaker = 0; + if (len == 1 && cxx_dialect >= cxx17 + && DECL_P (cand1->fn) + && DECL_COMPLETE_CONSTRUCTOR_P (cand1->fn) + && !(cand1->flags & LOOKUP_ONLYCONVERTING)) + { + bool elided1 = joust_maybe_elide_copy (cand1); + bool elided2 = joust_maybe_elide_copy (cand2); + /* As a tiebreaker below we will prefer a constructor to a conversion + operator exposed this way. */ + elided_tiebreaker = elided2 - elided1; + } + for (i = 0; i < len; ++i) { conversion *t1 = cand1->convs[i + off1]; @@ -10245,6 +10293,11 @@ joust (struct z_candidate *cand1, struct z_candidate *cand2, bool warn, if (winner) return winner; + /* Put this tiebreaker first, so that we don't try to look at second_conv of + a constructor candidate that doesn't have one. */ + if (elided_tiebreaker) + return elided_tiebreaker; + /* DR 495 moved this tiebreaker above the template ones. */ /* or, if not that, the context is an initialization by user-defined conversion (see diff --git a/gcc/testsuite/g++.dg/cpp0x/overload-conv-3.C b/gcc/testsuite/g++.dg/cpp0x/overload-conv-3.C index 42a135dbf44..ae587f9673b 100644 --- a/gcc/testsuite/g++.dg/cpp0x/overload-conv-3.C +++ b/gcc/testsuite/g++.dg/cpp0x/overload-conv-3.C @@ -17,5 +17,5 @@ struct Source { int main() { Source x; - Dest d(move(x)); // { dg-error "ambiguous" } + Dest d(move(x)); // { dg-error "ambiguous" "" { target c++14_down } } } diff --git a/gcc/testsuite/g++.dg/overload/conv-op2.C b/gcc/testsuite/g++.dg/overload/conv-op2.C index e8e533b1dc5..cc5ebe78619 100644 --- a/gcc/testsuite/g++.dg/overload/conv-op2.C +++ b/gcc/testsuite/g++.dg/overload/conv-op2.C @@ -1,5 +1,5 @@ // PR c++/81311 -// { dg-do link } +// { dg-do compile { target c++11 } } struct function { @@ -8,12 +8,12 @@ struct function struct ref { - operator function&() const; + operator function&() const = delete; } r; struct val { - operator function() const; + operator function() const = delete; } v; int main()
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