https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104873
Bug ID: 104873
Summary: Bug in overload resolution for constrained class
templates with deduction guides
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jgopel at gmail dot com
Target Milestone: ---
Given the following code:
```cpp
template<typename T>
concept C = true;
template<C T>
struct S {
S(const S&) { ... }
S(const T&) { ... }
};
// NOTE: The class is constrained, the deduction guide is not
template<typename T>
S(S<T>) -> S<S<T>>;
```
LLVM selects `S(S)` and GCC selects `S(T)`. I believe LLVM is correct here
because the built in deduction guide for `S(S)` is
```cpp
template<C T>
S(const S<T>&) -> S<T>;
```
which is more constrained. I don't believe there's a case where the code
example as-written is intended. I would suggest emitting a warning and using
the built in deduction guide.
Godbolt: https://godbolt.org/z/T79f164nW
LLVM issue suggesting a warning:
https://github.com/llvm/llvm-project/issues/54310
Relevant wording:
- https://eel.is/c++draft/over.match.class.deduct#1.3
- https://eel.is/c++draft/over.match.best#general-2.10
