[clang] clang fails to recognize a reference binding from prvalue with const qualification to rvalue ref during overloading resolution is ill-formed.

[changkhothuychung]

Given this program

struct Alloc{}; 

struct MyString
{
    MyString();
    MyString(MyString&&, const Alloc&); 
    MyString(MyString&&); 
    MyString(const MyString&, const Alloc&); 
    MyString(const MyString&); 
};

struct A
{
    operator const MyString() const; 
};

int main() {
    MyString s(A{}, Alloc{}); 
    return 0;
}

Clang reports error no viable conversion from 'A' to 'MyString', but gcc doesn't complain about it. The issue goes away when I change operator const to operator. Is this a bug in clang? If yes, I would like to work on this issue.

link to compiler explorer: https://godbolt.org/z/E7oP6hcP6

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nhat Nguyen (changkhothuychung)

Details

Given this program

struct Alloc{}; 

struct MyString
{
    MyString();
    MyString(MyString&&, const Alloc&); 
    MyString(MyString&&); 
    MyString(const MyString&, const Alloc&); 
    MyString(const MyString&); 
};

struct A
{
    operator const MyString() const; 
};

int main() {
    MyString s(A{}, Alloc{}); 
    return 0;
}

Clang reports error no viable conversion from 'A' to 'MyString', but gcc doesn't complain about it. The issue goes away when I change operator const to operator. Is this a bug in clang? If yes, I would like to work on this issue.

link to compiler explorer: https://godbolt.org/z/E7oP6hcP6

[changkhothuychung]

was using lldb to go through the program, since we have const operator const, so when we perform the user defined conversion from A, shouldn't ToType have const qualification in this line of code? https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaOverload.cpp#L1665

However, when I try to inspect isConstQualified of ToType, it returns false.

The overload MyString(MyString&&, const Alloc&) is selected in the viable list and finally selected as the best overload, but I think it shouldnt be in the viable list, as the conversion from const MyString to MyString&& is ill-formed.

[shafik]

Simpler reproducer: https://godbolt.org/z/nqMd4ExPv

struct Alloc{}; 

struct MyString {
    MyString(MyString&&, const Alloc&); 
    MyString(const MyString&, const Alloc&); 
};

struct A {
    operator const MyString() const; 
};

int main() {
    MyString s(A{}, Alloc{}); 
    return 0;
}

Removing the const from Alloc for the move case make it work: https://godbolt.org/z/bf3fhPTsc

struct Alloc{}; 

struct MyString {
    MyString(MyString&&,  Alloc&); 
    MyString(const MyString&, const Alloc&); 
};

struct A {
    operator const MyString() const; 
};

int main() {
    MyString s(A{}, Alloc{}); 
    return 0;
}

Looks somewhat related to: #65137

[changkhothuychung]

Simpler reproducer: https://godbolt.org/z/nqMd4ExPv

struct Alloc{};

struct MyString {
MyString(MyString&&, const Alloc&);
MyString(const MyString&, const Alloc&);
};

struct A {
operator const MyString() const;
};

int main() {
MyString s(A{}, Alloc{});
return 0;
}
Removing the const from Alloc for the move case make it work: https://godbolt.org/z/bf3fhPTsc

struct Alloc{};

struct MyString {
MyString(MyString&&, Alloc&);
MyString(const MyString&, const Alloc&);
};

struct A {
operator const MyString() const;
};

int main() {
MyString s(A{}, Alloc{});
return 0;
}
Looks somewhat related to: #65137

Oh yeah looks like its the same issue. I have been using lldb to run the program, and I see the move constructor overload is selected, there is an implicit conversion however I think it just checks whether an implicit conversion exists, and later on during initialization step, it fails because MyString&& r = A{} is ill-formed.

Changing A{} to {A{}} would work because now implicit copy constructor will be applied.

[changkhothuychung]

@shafik I found another case when clang fails but gcc succeeds.

struct A {
};

struct ConvertibleToConstA {
  operator const A();
};

int main()
{
    A a; 
    ConvertibleToConstA ca; 
    a = ca; 
    return 0; 
}

Link to compiler explorer - https://godbolt.org/z/79bWMscx9

This code is a test in file clang/test/SemaCXX/copy-assignment.cpp line 85.