https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101542
Bug ID: 101542 Summary: __gnu_cxx::sequence_buffer const copy constructor is badly broken Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: brooks at gcc dot gnu.org Target Milestone: --- This problem showed up when we were running the GCC 4.9.4 testsuite with the latest Clang trunk as the compiler, and found that the ext/rope/4.cc test started failing -- producing "wibblewibble" when it should produce "wibble". However, the problem is broader than that, and affects much more current GCC versions as well -- as can be seen at https://godbolt.org/z/74W78rdWP. Compiling the following program with Clang trunk produces "hellohello" as output: ---- #include <ext/rope> #include <iostream> template<typename T> T make_copy(const T &x) { return x; } int main() { std::string s; __gnu_cxx::sequence_buffer<std::string> a(s); { __gnu_cxx::sequence_buffer<std::string> b = a; b.push_back('h'); b.push_back('e'); b.push_back('l'); b.push_back('l'); b.push_back('o'); // Making a copy causes sequence_buffer to break. make_copy(b); } std::cout << s; } ---- This started failing with a recent Clang change (https://github.com/llvm/llvm-project/commit/7d2d5a3a6d7aaa40468c30250bf6b0938ef02c08), described as "Apply P1825 as Defect Report from C++11 up to C++20". See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1825r0.html for the defect report details. I would guess that GCC will be applying a similar change. My colleagues Jorge Gorbe and Richard Smith did a lot of digging on this, and Richard explains the problem as follows: ---- The problem with sequence_buffer is that the sequence_buffer(const sequence_buffer&) copy constructor copies the pending characters in the buffer to the new copy, and when both are destroyed, they get flushed to the underlying sequence twice. But sequence_buffer is more broken than that: it also has another constructor, sequence_buffer(sequence_buffer&) that has different semantics: it flushes the source first. So if you only ever use non-const sequence_buffer objects, never modify a copied-from object, and never do anything that would call the sequence_buffer(const sequence_buffer&) constructor, it will appear to work. And that's what this test was relying on. Part of the consequence of the Clang change is that in code like this: sequence_buffer f(sequence_buffer x) { return x; } ... the return statement unconditionally behaves like return (sequence_buffer&&)x;. And that means that we now choose the sequence_buffer(const sequence_buffer&) constructor, not the sequence_buffer(sequence_buffer&) constructor, because a non-const lvalue reference can't bind to an rvalue. Ultimately, I think that happens at line 342 of stl_algobase.h ("return __result"), here: https://gcc.gnu.org/git?p=gcc.git;a=blame;f=libstdc%2B%2B-v3/include/bits/stl_algobase.h;hb=9b61c47826156fe17fd5f4306470ade01e2fc4dc#l238. So: this libstdc++ code is broken, and the test really should never have passed. ---- In our case, we're simply xfailing the test since we don't have any code that uses __gnu_cxx::sequence_buffer or __gnu_cxx::rope, but I'm passing this bug report along in hopes that it's useful to the libstdc++ maintainers. :)