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From: "wuyongwei at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/98283] New: decltype(auto) may be deduce static data member to wrong type Date: Tue, 15 Dec 2020 01:13:57 +0000 [thread overview] Message-ID: <bug-98283-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98283 Bug ID: 98283 Summary: decltype(auto) may be deduce static data member to wrong type Product: gcc Version: 10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: wuyongwei at gmail dot com Target Milestone: --- The following code shows that `decltype(auto)` is deduced to the wrong type when used in template code for an expression like `(var.static_data_member>)`. ```cpp #include <cstddef> #include <type_traits> #include <utility> template <typename T> struct type_displayer; #define TYPE_DISPLAY(x) type_displayer<decltype(x)> test_obj; struct SkipField {}; struct TestSkip { template <typename T, std::size_t> struct FIELD; int value; static SkipField unused; template <typename T> struct FIELD<T, 0> { T &&obj; FIELD(T &&ref) : obj(std::forward<T>(ref)) { } decltype(auto) value() { return (obj.value); } static constexpr const char *name() { return "value"; } }; template <typename T> struct FIELD<T, 0 + 1> { T &&obj; FIELD(T &&ref) : obj(std::forward<T>(ref)) { } decltype(auto) value() { return (obj.unused); } decltype((obj.unused)) valueAlt() { return (obj.unused); } static constexpr const char *name() { return "unused"; } }; }; int main() { TestSkip s; // decltype((TestSkip::FIELD<TestSkip&, 0>(s).value())) is int& // decltype((TestSkip::FIELD<TestSkip&, 1>(s).value())) is SkipField, but it should be SkipField& // decltype((TestSkip::FIELD<TestSkip&, 1>(s).valueAlt())) is SkipField& } ``` This is a simplified usage scenario, where I want to use a macro to generate code to emulate static reflection. Here is the problematic part: ```cpp decltype(auto) value() { return (obj.unused); } ``` This function returns `SkipField`, instead of the expected `SkipField&`. I have verified that clang does what I want. Currently I have to avoid `decltype(auto)` as a workaround: ```cpp decltype((obj.unused)) valueAlt() { return (obj.unused); } ``` (I used the macro TYPE_DISPLAY to check the type of an expression.)
next reply other threads:[~2020-12-15 1:13 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-12-15 1:13 wuyongwei at gmail dot com [this message] 2021-08-03 5:33 ` [Bug c++/98283] decltype(auto) may deduce a static data member to wrong type in a template pinskia at gcc dot gnu.org 2023-05-07 15:58 ` cvs-commit at gcc dot gnu.org 2023-05-07 15:58 ` ppalka at gcc dot gnu.org
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