clang trunk does not compile overloaded member functions with ref-qualifier

[forworldm]

struct C {
    template<int> void f() && {}
    template<typename T> void f() const {}
};

<source>:3:31: error: cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&&'
    3 |     template<typename T> void f() const {}
      |                               ^
<source>:2:24: note: previous declaration is here
    2 |     template<int> void f() && {}
      |                        ^
1 error generated.
Compiler returned: 1

see https://godbolt.org/z/Kv83s6oY3

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: forworldm (forworldm)

```c++ struct C { template<int> void f() && {} template<typename T> void f() const {} }; ``` ``` <source>:3:31: error: cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&&' 3 | template<typename T> void f() const {} | ^ <source>:2:24: note: previous declaration is here 2 | template<int> void f() && {} | ^ 1 error generated. Compiler returned: 1 ```

see https://godbolt.org/z/Kv83s6oY3

[DimitryAndric]

This regressed as a side effect of llvmorg-18-init-07480-gaf47517 ("[C++] Implement "Deducing this" (P0847R7)") by @cor3ntin (cc @AaronBallman).

I think it should still accept the construction used here? It always worked in the past, and appears to break even compilation of older llvm versions!

[DimitryAndric]

Specifically, it seems to be caused by the moving of the block in IsOverloadOrOverrideImpl at https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaOverload.cpp#L1346, that has:

// If the function is a class member, its signature includes the
// cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.

and emits err_ref_qualifier_overload in this case. The block used to be in Sema::IsOverload() and was subtly changed, I guess.

[cor3ntin]

We should probably check the template parameter list before the check of signature as two functions with different template head are not overloads

[DimitryAndric]

How problematic is it to temporarily revert af47517, until this problem can be fixed? I'm encountering the cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&&' error in quite a few packages, for example SciPy (from https://scipy.org/).

[cor3ntin]

Pretty problematic and disruptive given the fix should be easy... I'll get on it...

[DimitryAndric]

Thanks, that seems to work fine!