[BUG] Recognize non-deducible parameter

[JohelEGP]

Title: Use cpp2::in for non-deducible parameters.

Description:

Currently, Cppfront doesn't wrap in cpp2::in
a template function $F$'s function parameter
that has the type of one of $F$'s template parameter
to avoid making it non-deducible.

However, it also does the same
when a similar use of $F$'s template parameter
makes the function parameter non-deducible.

If a function parameter is already non-deducible,
its type should be wrapped in cpp2::in.

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

f: <T> (x: std::integral_constant<i32, T::value>) = { }
main: () = { }

Commands:

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

Expected result:

template<typename T> auto f(cpp2::in<std::integral_constant<cpp2::i32,T::value>> x) -> void{}

Actual result and error:

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> const& x) -> void{}

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"



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

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> const& x) -> void;
auto main() -> int;


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

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> const& x) -> void{}
auto main() -> int{}

Output:

Program returned: 0

[JohelEGP]

Title: Recognize non-deducible parameter.

Description:

The report generalizes.
The same applies to out parameters.

Minimal reproducer (https://cpp2.godbolt.org/z/Tnvczs8qa):

f: <T> (out x: std::integral_constant<i32, T::value>) = { x = (); }
main: () = {
  t: type == std::integral_constant<i32, 0>;
  i: t;
  f<t>(out i);
}

Commands:

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

Expected result:

template<typename T> auto f(cpp2::out<std::integral_constant<cpp2::i32, T::value>> x) -> void;

Actual result and error:

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> x_) -> void;

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"



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

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> x_) -> void;
auto main() -> int;
  

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

template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> x_) -> void{
                                                          auto x = cpp2::out(x_); 
x.construct(); }
auto main() -> int{
  using t = std::integral_constant<cpp2::i32,0>;
  cpp2::deferred_init<t> i; 
  f<t>(cpp2::out(&i));
}

Output:

main.cpp2:5:3: error: no matching function for call to 'f'
    5 |   f<t>(cpp2::out(&i));
      |   ^~~~
main.cpp2:1:27: note: candidate function template not viable: no known conversion from 'cpp2::out<integral_constant<int, 0>>' (aka 'cpp2::out<std::integral_constant<int, 0>>') to 'std::integral_constant<cpp2::i32, t::value>' (aka 'integral_constant<int, t::value>') for 1st argument
    5 | template<typename T> auto f(std::integral_constant<cpp2::i32,T::value> x_) -> void{
      |                           ^
    6 |                                                           auto x = cpp2::out(x_); 
    7 | #line 1 "/app/main.cpp2"
    8 | x.construct(); }
    9 | auto main() -> int{
   10 |   using t = std::integral_constant<cpp2::i32,0>;
   11 |   cpp2::deferred_init<t> i; 
   12 |   f<t>(cpp2::out(&i));
      |        ~~~~~~~~~~~~~
1 error generated.

[JohelEGP]

See #533 (comment) for the source of the bug.