Maybe a bug of concept and vector

[scarsty]

These codes cannot be complied with clang 19, but of no problem with MSVC and GCC.

#include <iostream>
#include <string>
#include <vector>

class A
{
    std::string a;
    std::vector<A> v;

public:
    A(int i) { a = std::to_string(i); }

    int toInt() const { return atoi(a.c_str()); }

    void pushBack(const A& a) { v.push_back(a); }    // line 1

    template <typename T>
        requires requires(const A& a, T& t) { A2T(a, t); }    // line 2
    operator T() const
    {
        T t;
        A2T(*this, t);
        return t;
    }
};

inline void A2T(const A& a, int& f) { f = a.toInt(); }

int main()
{
    A a(20);
    int f = a;
    std::cout << f << std::endl;
    return 0;
}

The error is

<source>:43:9: error: no viable conversion from 'A' to 'int'
   43 |     int f = a;
      |         ^   ~
<source>:23:5: note: candidate template ignored: constraints not satisfied [with T = int]
   23 |     operator T() const
      |     ^
<source>:22:47: note: because 'A2T(a, t)' would be invalid: use of undeclared identifier 'A2T'
   22 |         requires requires(const A& a, T& t) { A2T(a, t); }
      |                                               ^
1 error generated.

But if one of the line 1 or 2 is removed, things become OK. I guess it might be a bug of concept, does anyone have other idea?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: SunTY (scarsty)

These codes cannot be complied with clang 19, but of no problem with MSVC and GCC.

#include <iostream>
#include <string>
#include <vector>

class A
{
    std::string a;
    std::vector<A> v;

public:
    A(int i) { a = std::to_string(i); }

    int toInt() const { return atoi(a.c_str()); }

    void pushBack(const A& a) { v.push_back(a); }    // line 1

    template <typename T>
        requires requires(const A& a, T& t) { A2T(a, t); }    // line 2
    operator T() const
    {
        T t;
        A2T(*this, t);
        return t;
    }
};

inline void A2T(const A& a, int& f) { f = a.toInt(); }

int main()
{
    A a(20);
    int f = a;
    std::cout << f << std::endl;
    return 0;
}

The error is

<source>:43:9: error: no viable conversion from 'A' to 'int'
   43 |     int f = a;
      |         ^   ~
<source>:23:5: note: candidate template ignored: constraints not satisfied [with T = int]
   23 |     operator T() const
      |     ^
<source>:22:47: note: because 'A2T(a, t)' would be invalid: use of undeclared identifier 'A2T'
   22 |         requires requires(const A& a, T& t) { A2T(a, t); }
      |                                               ^
1 error generated.

But if one of the line 1 or 2 is removed, things become OK. I guess it might be a bug of concept, does anyone have other idea?

[shafik]

edg agrees w/ clang: https://godbolt.org/z/9oM5sdfj8

Using --stdlib=libc++ does not change the results.

[cor3ntin]

It looks like lookup in requires expression doesn't happen in the correct context