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From: Thomas Schwinge <tschwinge@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc/devel/rust/master] c++: Quash -Wdangling-reference for member operator* [PR107488] Date: Wed, 14 Dec 2022 15:40:23 +0000 (GMT) [thread overview] Message-ID: <20221214154023.60682382AB65@sourceware.org> (raw) https://gcc.gnu.org/g:5c2e6c93d919b939ba7eca2100fa35cc444f4b8d commit 5c2e6c93d919b939ba7eca2100fa35cc444f4b8d Author: Marek Polacek <polacek@redhat.com> Date: Tue Nov 1 17:05:52 2022 -0400 c++: Quash -Wdangling-reference for member operator* [PR107488] -Wdangling-reference complains here: std::vector<int> v = ...; std::vector<int>::const_iterator it = v.begin(); while (it != v.end()) { const int &r = *it++; // warning } because it sees a call to __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator* which returns a reference and its argument is a TARGET_EXPR representing the result of __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator++ But 'r' above refers to one of the int elements of the vector 'v', not to a temporary object. Therefore the warning is a false positive. I suppose code like the above is relatively common (the warning broke cppunit-1.15.1 and a few other projects), so presumably it makes sense to suppress the warning when it comes to member operator*. In this case it's defined as reference operator*() const _GLIBCXX_NOEXCEPT { return *_M_current; } and I'm guessing a lot of member operator* are like that, at least when it comes to iterators. I've looked at _Fwd_list_iterator, _Fwd_list_const_iterator, __shared_ptr_access, _Deque_iterator, istream_iterator, etc, and they're all like that, so adding #pragmas would be quite tedious. :/ PR c++/107488 gcc/cp/ChangeLog: * call.cc (do_warn_dangling_reference): Quash -Wdangling-reference for member operator*. gcc/testsuite/ChangeLog: * g++.dg/warn/Wdangling-reference5.C: New test. (cherry picked from commit 32a06ce38a38bf37db468f0e6c83520fcc221534) Diff: --- gcc/cp/call.cc | 12 +++++++++++- gcc/testsuite/g++.dg/warn/Wdangling-reference5.C | 22 ++++++++++++++++++++++ 2 files changed, 33 insertions(+), 1 deletion(-) diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index c7c7a122045..2c0fa37f53a 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -13467,7 +13467,17 @@ do_warn_dangling_reference (tree expr) can be e.g. const int& z = std::min({1, 2, 3, 4, 5, 6, 7}); which doesn't dangle: std::min here returns an int. */ - || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl)))) + || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl))) + /* Don't emit a false positive for: + std::vector<int> v = ...; + std::vector<int>::const_iterator it = v.begin(); + const int &r = *it++; + because R refers to one of the int elements of V, not to + a temporary object. Member operator* may return a reference + but probably not to one of its arguments. */ + || (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) + && DECL_OVERLOADED_OPERATOR_P (fndecl) + && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF))) return NULL_TREE; /* Here we're looking to see if any of the arguments is a temporary diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C new file mode 100644 index 00000000000..59b5538aee5 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C @@ -0,0 +1,22 @@ +// PR c++/107488 +// { dg-do compile } +// { dg-options "-Wdangling-reference" } + +#include <vector> + +int +do_sum (std::vector<int>& v) +{ + int sum = 0; + + std::vector<int>::const_iterator it = v.begin(); + while (it != v.end()) + { + // R refers to one of the int elements of V, not to a temporary + // object, so no dangling reference here. + const int &r = *it++; // { dg-bogus "dangling reference" } + sum += r; + } + + return sum; +}
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