<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/106428>106428</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] std::invoke() substitution failure when using function object with default argument
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          tcbrindle
      </td>
    </tr>
</table>

<pre>
    The following test program compiles with libc++17 and libstdc++, but fails with libc++18:

```cpp
#include <concepts>
#include <functional>
#include <ranges>
#include <source_location>

#define FWD(x) static_cast<decltype(x)&&>(x)

struct add_fn {
    template <std::integral T>
    constexpr auto operator()(T lhs, T rhs,
 std::source_location loc = std::source_location::current())
        const -> T
    {
 return lhs + rhs;
    }
};

inline constexpr auto add = add_fn{};

struct fold_op {
    template <typename Rng, typename Func, typename Init>
    constexpr auto operator()(Rng&& rng, Func func, Init init) const
    {
        auto init_ = std::ranges::range_value_t<Rng>(std::move(init));
        for (auto&& elem : rng) {
            init_ = std::invoke(func, std::move(init_), FWD(elem));
        }
        return init_;
 }
};

inline constexpr auto fold = fold_op{};


auto sum(std::span<int const> arr) -> int
{
    return fold(arr, add, 0);
}
```

https://godbolt.org/z/h47dnM3eP

The error is:

```
<source>:27:21: error: no matching function for call to 'invoke'
   27 |             init_ = std::invoke(func, std::move(init_), FWD(elem));
      |                     ^~~~~~~~~~~
<source>:38:16: note: in instantiation of function template specialization 'fold_op::operator()<std::span<const int> &, add_fn, int>' requested here
 38 |     return fold(arr, add, 0);
      | ^
/opt/compiler-explorer/clang-18.1.0/bin/../include/c++/v1/__functional/invoke.h:27:1: note: candidate template ignored: substitution failure [with _Fn = add_fn &, _Args = <__libcpp_remove_reference_t<int &>, const int &>]
   27 | invoke(_Fn&& __f, _Args&&... __args) noexcept(is_nothrow_invocable_v<_Fn, _Args...>) {
      | ^
1 error generated.
Compiler returned: 1
```

Removing the `source_location` default argument from `add_fn::operator()` allows the code to compile as expected.



</pre>
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