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From: Jakub Jelinek <jakub@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r12-6516] c++: Silence -Wuseless-cast warnings during move [PR103480] Date: Wed, 12 Jan 2022 08:49:21 +0000 (GMT) [thread overview] Message-ID: <20220112084921.847D83858001@sourceware.org> (raw) https://gcc.gnu.org/g:6bba184ccbf47368eaea27ee2c1e7b850526640b commit r12-6516-g6bba184ccbf47368eaea27ee2c1e7b850526640b Author: Jakub Jelinek <jakub@redhat.com> Date: Wed Jan 12 09:47:46 2022 +0100 c++: Silence -Wuseless-cast warnings during move [PR103480] This is maybe just a shot in the dark, but IMHO we shouldn't be diagnosing -Wuseless-cast on casts the compiler adds on its own when calling its move function. We don't seem to warn when user calls std::move either. We call move on elinit (*NON_LVALUE_EXPR <(struct C[2] &&) &D.2497->b>)[0] so it is already an xvalue_p and try to static_cast it to struct C &&. But we don't warn e.g. on std::move (std::move (whatever)). Fixed by not doing the static cast and just returning expr from move if expr is already an xvalue. 2022-01-11 Jakub Jelinek <jakub@redhat.com> Jason Merrill <jason@redhat.com> PR c++/103480 * tree.c (move): If expr is xvalue_p, just return expr without build_static_cast. * g++.dg/warn/Wuseless-cast2.C: New test. Diff: --- gcc/cp/tree.c | 2 ++ gcc/testsuite/g++.dg/warn/Wuseless-cast2.C | 24 ++++++++++++++++++++++++ 2 files changed, 26 insertions(+) diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c index d0c6490e42f..7f7de86b4e8 100644 --- a/gcc/cp/tree.c +++ b/gcc/cp/tree.c @@ -1304,6 +1304,8 @@ move (tree expr) { tree type = TREE_TYPE (expr); gcc_assert (!TYPE_REF_P (type)); + if (xvalue_p (expr)) + return expr; type = cp_build_reference_type (type, /*rval*/true); return build_static_cast (input_location, type, expr, tf_warning_or_error); diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C new file mode 100644 index 00000000000..22e403973e7 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C @@ -0,0 +1,24 @@ +// PR c++/103480 +// { dg-do compile { target c++14 } } +// { dg-options "-Wuseless-cast" } + +template <typename T, int N> +struct A { typedef T t[N]; }; +template <typename T, int N> +struct B { typename A<T, N>::t b; }; +struct C { + constexpr C (C &&) {} + template <int N> + static auto bar () + { + B<C, N> r; + return r; // { dg-bogus "useless cast to type" } + } + C () = default; +}; + +void +foo () +{ + C::bar<2> (); +}
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