[BUG] Parameter direction for template parameter is ignored

[JohelEGP]

Title: Parameter direction for template parameter is ignored.

Minimal reproducer (https://cpp2.godbolt.org/z/M1jxGdeqe):

a: <in T> type = { }
b: <in V: _> type = { }
main: () = { }

Commands:

cppfront -clean-cpp1 main.cpp2
clang++17 -std=c++2b -stdlib=libc++ -lc++abi -I . main.cpp

Expected result:

For the : type case, a diagnostic.
For the : _ case, adherence to the specified parameter direction.

template<typename T> class a; // A Cpp2 diagnostic here.
template<const auto& V> class b;

Actual result and error:

template<typename T> class a;
template<auto V> class b;

Program returned: 0

Cpp2 lowered to Cpp1.

#include "cpp2util.h"

template<typename T> class a;
template<auto V> class b;

template<typename T> class a {
      public: a() = default;
      public: a(a const&) = delete; /* No 'that' constructor, suppress copy */
      public: auto operator=(a const&) -> void = delete;

};
template<auto V> class b {
      public: b() = default;
      public: b(b const&) = delete; /* No 'that' constructor, suppress copy */
      public: auto operator=(b const&) -> void = delete;

};
auto main() -> int;

auto main() -> int{}

[realgdman]

Possibly related: I'm using library (flecs) with API like

ecs.system<Position, const Velocity>()
    .each([] some lambda

And haven't found way to specify that const in template call to system()
Which, I believe, cannot be deduced from parameters of lambda

	foo<int>();
	foo<const int>(); //error
	foo<int const>(); //error
	foo<in int>(); //error

https://cpp2.godbolt.org/z/sjMWTq3b7

[JohelEGP]

Seems like a bug: https://cpp2.godbolt.org/z/Tx8e9KfKc.
I suggest you open a new issue, since this one is about template parameters, not template arguments.

[realgdman]

Ok, I do later, what I mean by related is, should I be able to put in in <> in that case.

[JohelEGP]

I don't think that can work.
In a subexpression, a parameter direction operates on an expression, not a type.
cppfront doesn't have enough semantic information for f<in int>() to work.

[hsutter]

Parameter direction for template parameter is ignored.

Yes, this is by design -- I don't know what it would mean.

I should probably emit a diagnostic for it though. Thanks!