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.)
