From 6bba184ccbf47368eaea27ee2c1e7b850526640b Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Wed, 12 Jan 2022 09:47:46 +0100 Subject: [PATCH] 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 Jason Merrill 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. --- gcc/cp/tree.c | 2 ++ gcc/testsuite/g++.dg/warn/Wuseless-cast2.C | 24 ++++++++++++++++++++++++ 2 files changed, 26 insertions(+) create mode 100644 gcc/testsuite/g++.dg/warn/Wuseless-cast2.C diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c index d0c6490..7f7de86 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 0000000..22e4039 --- /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 +struct A { typedef T t[N]; }; +template +struct B { typename A::t b; }; +struct C { + constexpr C (C &&) {} + template + static auto bar () + { + B r; + return r; // { dg-bogus "useless cast to type" } + } + C () = default; +}; + +void +foo () +{ + C::bar<2> (); +} -- 2.7.4