[BUG] Support _ unnamed template parameter.

[JohelEGP]

Title: Support _ unnamed template parameter.

Description:

Commit 6ce2643 added support only for function parameters.

Minimal reproducer (https://cpp2.godbolt.org/z/WY56cMoTd):

f: <_, _> (_, _) = { }
main: () = { }

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cpp

Expected result:

The template parameters to be named similar to the function parameters.

template<typename tparam1, typename tparam2> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void;

Actual result and error:

template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void;

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"



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

template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void;
auto main() -> int;


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

template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void{}
auto main() -> int{}

Output:

build/main.cpp:12:31: error: declaration of '_' shadows template parameter
   12 | template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void;
      |                               ^
build/main.cpp:12:19: note: template parameter is declared here
   12 | template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void;
      |                   ^
build/main.cpp:18:31: error: declaration of '_' shadows template parameter
   18 | template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void{}
      |                               ^
build/main.cpp:18:19: note: template parameter is declared here
   18 | template<typename _, typename _> auto f([[maybe_unused]] auto const& param1, [[maybe_unused]] auto const& param2) -> void{}
      |                   ^
2 errors generated.

See also:

• Commit 6ce2643.

[gregmarr]

I'm curious what the use case is for unused template parameters. Is it for specializing an existing template?

[JohelEGP]

Compilers don't seem to warn on these.
But I have to invent names,
and maybe I'll run out of T, U, V, W, ???.
Of course, this mostly happens in tests.

[gregmarr]

Okay, thanks.

[hsutter]

Thanks!

[JohelEGP]

This is now #877.

Repeat of #877

It doesn't work for NTTPs: https://cpp2.godbolt.org/z/rdb6dbvv1.
Also, <_...: _> doesn't lower to a parameter pack declaration.

t: @struct <_...: _> type = {
  f: <_: int> () -> i32 = 0;
}
main: () = { }

template<auto _> class t {
  public: template<int _> [[nodiscard]] static auto f() -> cpp2::i32;
};

main.cpp2:2:24: error: declaration of '_' shadows template parameter
    2 |   public: template<int _> [[nodiscard]] static auto f() -> cpp2::i32;
      |                        ^
main.cpp2:1:15: note: template parameter is declared here
    1 | template<auto _> class t {
      |               ^