https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116379
Bug ID: 116379
Summary: Type deduction error on decltype(auto)
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: 1963096826 at qq dot com
Target Milestone: ---
GCC type deduction incorrect in special circumstances. This problem persisted
until the gcc(trunk) version on [Goldbolt](https://gcc.godbolt.org/)
The following simple code cannot be compiled in gcc.
(see on goldbot: https://gcc.godbolt.org/z/c81fra83z)
```cpp
#include <type_traits>
#include <utility>
template<typename T>
struct X{
T val;
decltype(auto) value(){
return (val);
}
};
int main(){
int i = 0;
X<int&&> x{ std::move(i) };
static_assert(std::is_same_v<decltype(x.value()), int&>);
}
```
gcc output:
```
<source>: In instantiation of 'decltype(auto) X<T>::value() [with T = int&&]':
<source>:15:50: required from here
15 | static_assert(std::is_same_v<decltype(x.value()), int&>);
| ~~~~~~~^~
<source>:8:20: error: cannot bind rvalue reference of type 'int&&' to lvalue of
type 'int'
8 | return (val);
| ^
<source>: In function 'int main()':
<source>:15:24: error: static assertion failed
15 | static_assert(std::is_same_v<decltype(x.value()), int&>);
| ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
Here decltype (auto) should have been deduced as int&, but it was incorrectly
deduced as int&&.
Details on how to trigger this bug：
1.If the type of `val` is changed from `T` to `T&&`(There should be no
difference between the two here, both are int&&), this bug will not be
triggered.
2.If the type of `val` is not a template parameter, this bug will not be
triggered.
3.If change `return (val)` to `return (this->val)`，this bug will not be
triggered.
