[BUG] Type of `this` can't use the current type beyond its full name

[JohelEGP]

Title: Type of this can't be a nested type.

Description:

It's common to alias complicated base types.
Cpp2's member declaration order suggests a nested type could be used for a this member.
But it's not generally possible for the lowered Cpp1 to make use of it
(it's directly impossible,
and only for the simple cases could cppfront recursively replace nested types for their initializer).

Minimal reproducer (https://cpp2.godbolt.org/z/vs1abM95E):

#include <chrono>
my_time: type = {
  this: duration;
  rep: type == cpp2::longdouble;
  period: type == std::ratio<17, 29>;
  duration: type == std::chrono::duration<rep, period>;
  operator=: (out this, d: duration) = { duration = d; }
}
main: () = { }

Commands:

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

Expected result:

A well-formed program or a diagnostic by cppfront.

Actual result and error:

Cpp2 lowered to Cpp1.

#include "cpp2util.h"


class my_time;

#include <chrono>
class my_time: public duration {

  public: using rep = cpp2::longdouble;
  public: using period = std::ratio<17,29>;
  public: using duration = std::chrono::duration<rep,period>;
  public: explicit my_time(cpp2::in<duration> d);

  public: my_time(my_time const&) = delete; /* No 'that' constructor, suppress copy */
  public: auto operator=(my_time const&) -> void = delete;
};
auto main() -> int;

  my_time::my_time(cpp2::in<duration> d)
                                         : duration{ d }
  {}

auto main() -> int{}

Output.

build/_cppfront/main.cpp:9:23: error: expected class name
class my_time: public duration {
                      ^

[realgdman]

Is this supposed to work? I can't get it to work in cpp1 either. But if aliases/using moved outside, before class, its fine.
https://cpp.godbolt.org/z/cKb38erdo
https://cpp2.godbolt.org/z/xrYe7K9ab

[JohelEGP]

If you put a type alias before a this member, it generates bad code:

my_time: type = {
  rep: type == cpp2::longdouble;
  this: duration;
}

class my_time {
  public: using rep = cpp2::longdouble;
  : public duration

The code in the OP is wrong.
Nested types aren't order-independent: https://compiler-explorer.com/z/7761n18Ej.

[JohelEGP]

The issue is also relevant when the type of a member-as-base uses the type's template parameters.
From commit 7e80110:

t: @struct <T: type> type = { // clang-format on
  // Emits `struct t_i32_as_base { typename T::value_type i32; };`,
  // errors with `‘T’ has not been declared`.
  // i32: T::value_type = ();
  this: T::type = ();
}

[JohelEGP]

Title: Type of this member can't be a template parameter of the current type.

Description:

See #478 (comment), which also applies to the type.

Minimal reproducer (https://cpp2.godbolt.org/z/o5YE6n34f):

t: @struct <T: type> type = {
  x: T = 0;
  this: std::monostate = ();
}
main: () = { _ = :t<i32> = (); }

Commands:

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

Expected result: I don't know. Maybe make the emitted base type a template.

Actual result and error:

main.cpp2:7:22: error: unknown type name 'T'
    7 | struct t_x_as_base { T x; };
      |                      ^
1 error generated.

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"

template<typename T> class t;


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


struct t_x_as_base { T x; };
template<typename T> class t: public t_x_as_base, public std::monostate {

};
auto main() -> int;


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


auto main() -> int{(void) t<cpp2::i32>{}; }

[JohelEGP]

I have changed the issue's title to

[BUG] Type of this can't use the current type beyond its full name

This works (https://cpp2.godbolt.org/z/xzr8G1roe):

t: @struct <T: type> type = {
  this: std::ranges::view_interface<t<T>> = ();
}
main: () = { _ = :t<i32> = (); }

But not attempting to use t as an injected-class-name (https://cpp2.godbolt.org/z/EfEErWMe6):

t: @struct <T: type> type = {
  this: std::ranges::view_interface<t> = ();
}
main: () = { _ = :t<i32> = (); }

main.cpp2:1:66: error: use of class template 't' requires template arguments
    1 | template<typename T> class t: public std::ranges::view_interface<t> {
      |                                                                  ^

That's why I use "full name".

[JohelEGP]

Members before bases also can't use template parameters.
With the algorithms from #533, it'd be possible to fix that.
See https://cpp2.godbolt.org/z/vE3zohhfG:

t: @struct <T: type> type = {
  x: T;
  this: T;
}
main: () = { }

struct t_x_as_base { T x; };
template<typename T> class t: public t_x_as_base, public T {

};

main.cpp2:7:22: error: 'T' does not name a type

[JohelEGP]

This is similar in theme to #704.
In Cpp1, the expectation is that you can use names in a scope that are declared
(or will be in type scope; limitations apply).
That declaration could be in a previous statement, or lexically earlier in the current statement.
Because Cppfront lowers Cpp2 to Cpp1, the same expectation can fail to apply to Cpp2 source code.

[JohelEGP]

#471 (comment) now segfaults: https://cpp2.godbolt.org/z/qc8v8h4Wh.