[BUG] No support for multi-argument indexing

[JohelEGP]

Title: No support for multi-argument indexing.

Description:

An expression like x[y,z]
lowers to cpp2::assert_in_bounds(x, y, z),
but has an arity of 2.

Minimal reproducer (https://cpp2.godbolt.org/z/TPoGe1czc):

t: type = {
  operator[]: (inout this, x: _, y: _) -> i32 = 42;
}
main: () = {
  [[assert: t()[1, 2] == 42]]
}

Commands:

cppfront -clean-cpp1 main.cpp2
clang++17 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -I . main.cpp

Expected result:

It doesn't seem trivial to support multi-argument indexing in cpp2::assert_in_bounds.
See https://en.cppreference.com/w/cpp/container/mdspan/operator_at.

  cpp2::Default.expects(t()[1, 2]==42, "");

Actual result and error:

  cpp2::Default.expects(cpp2::assert_in_bounds(t(), 1, 2)==42, "");

Cpp2 lowered to Cpp1.

#include "cpp2util.h"

class t;
  
class t {
  public: [[nodiscard]] auto operator[](auto const& x, auto const& y) -> cpp2::i32;

  public: t() = default;
  public: t(t const&) = delete; /* No 'that' constructor, suppress copy */
  public: auto operator=(t const&) -> void = delete;
};
auto main() -> int;
  

  [[nodiscard]] auto t::operator[](auto const& x, auto const& y) -> cpp2::i32 { return 42;  }

auto main() -> int{
  cpp2::Default.expects(cpp2::assert_in_bounds(t(), 1, 2)==42, "");
}

Output.

build/_cppfront/main.cpp:20:25: error: no matching function for call to 'assert_in_bounds'
  cpp2::Default.expects(cpp2::assert_in_bounds(t(), 1, 2)==42, "");
                        ^~~~~~~~~~~~~~~~~~~~~~
raw.githubusercontent.com/hsutter/cppfront/main/include/cpp2util.h:399:6: note: candidate function template not viable: requires 2 arguments, but 3 were provided
auto assert_in_bounds(auto&& x, auto&& arg CPP2_SOURCE_LOCATION_PARAM_WITH_DEFAULT) -> decltype(auto)
     ^
raw.githubusercontent.com/hsutter/cppfront/main/include/cpp2util.h:407:6: note: candidate function template not viable: requires 2 arguments, but 3 were provided
auto assert_in_bounds(auto&& x, auto&& arg CPP2_SOURCE_LOCATION_PARAM_WITH_DEFAULT) -> decltype(auto)
     ^