[BUG] fix(reflect): recognize default constructor overload in `@basic_value`

[JohelEGP]

Title: fix(reflect): recognize default constructor overload in @basic_value.

Description:

Now that we have default function arguments,
it's possible for a default constructor
to overload with a constructor with default arguments.

Minimal reproducer (https://cpp2.godbolt.org/z/jrs6YYbab):

lineno_t: type == std::int32_t;
colno_t: type == std::int32_t;   // not int16_t... encountered >80,000 char line during testing
index_t: type == std::int32_t;
source_position: @value type =
{
    public lineno: lineno_t = 1;   // one-based offset into program source
    public colno: colno_t = 1;     // one-based offset into line

    operator=: (implicit out this, l: lineno_t = 1, c: colno_t = 1) =
    {
        lineno = l;
        colno = c;
    }

    to_string: (this) -> std::string = "((lineno)$,(colno)$)";
}
main: () = {
  _ = :source_position = ();
}

Commands:

cppfront main.cpp2
clang++-18 -std=c++26 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -Werror=unused-value -Werror=unused-parameter -Werror=unused-variable -I . main.cpp

Expected result: The default constructor not to be overloaded.

Actual result and error:

Cpp2 lowered to Cpp1:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"


class source_position;


//=== Cpp2 type definitions and function declarations ===========================

using lineno_t = std::int32_t;
using colno_t = std::int32_t;    // not int16_t... encountered >80,000 char line during testing
using index_t = std::int32_t;
class source_position
 {
    public: lineno_t lineno {1};   // one-based offset into program source
    public: colno_t colno {1};     // one-based offset into line

    public: source_position(cpp2::impl::in<lineno_t> l = 1, cpp2::impl::in<colno_t> c = 1);

    public: [[nodiscard]] auto to_string() const& -> std::string;
    public: [[nodiscard]] auto operator<=>(source_position const& that) const& -> std::strong_ordering = default;
public: source_position(source_position const& that);

public: auto operator=(source_position const& that) -> source_position& ;
public: source_position(source_position&& that) noexcept;
public: auto operator=(source_position&& that) noexcept -> source_position& ;
public: explicit source_position();

};
auto main() -> int;

//=== Cpp2 function definitions =================================================


    source_position::source_position(cpp2::impl::in<lineno_t> l, cpp2::impl::in<colno_t> c)
        : lineno{ l }
        , colno{ c }
    {

    }

    [[nodiscard]] auto source_position::to_string() const& -> std::string { return "(" + cpp2::to_string(lineno) + "," + cpp2::to_string(colno) + ")";  }


    source_position::source_position(source_position const& that)
                                : lineno{ that.lineno }
                                , colno{ that.colno }{}

auto source_position::operator=(source_position const& that) -> source_position& {
                                lineno = that.lineno;
                                colno = that.colno;
                                return *this;}
source_position::source_position(source_position&& that) noexcept
                                : lineno{ std::move(that).lineno }
                                , colno{ std::move(that).colno }{}
auto source_position::operator=(source_position&& that) noexcept -> source_position& {
                                lineno = std::move(that).lineno;
                                colno = std::move(that).colno;
                                return *this;}
source_position::source_position(){}
auto main() -> int{
  static_cast<void>(source_position{});
}

Output:

main.cpp2:18:21: error: call to constructor of 'source_position' is ambiguous
   18 |   static_cast<void>(source_position{});
      |                     ^              ~~
main.cpp2:9:22: note: candidate constructor
    9 |     source_position::source_position(cpp2::impl::in<lineno_t> l, cpp2::impl::in<colno_t> c)
      |                      ^
main.cpp2:33:18: note: candidate constructor
   33 | source_position::source_position(){}
      |                  ^
1 error generated.

See also:

• Commit 873b760.