[BUG] `forward` of capture `$` crashes compiler

[realgdman]

Describe the bug
Enclosing capture in forward crashes compiler

Reproduction Code

main: ()   [[post: (forward c$) == 5]] //error
= {
	c := 5;
	:() = { (forward c$); }; //error	
	:() = { foo(forward c$); } (); //error - possibly valid usecase	
}

foo: (forward x) = std::cout << x;

https://cpp2.godbolt.org/z/E4rqKW4cn

Version
latest (38aec57)

Command lines
cppfront/cppfront $1.cpp2 -p
clang++-15 -Icppfront/include $1.cpp -std=c++20 -o $1

Expected result
Diagnostic, probably disallowing forward with capture

Actual result
cppfront: source/cppfront.cpp:2712: void cpp2::cppfront::emit(cpp2::postfix_expression_node &, bool): Assertion `!current_args.back().ptoken' failed.
Aborted (core dumped)

Additional context
Assert line is https://github.com/hsutter/cppfront/blob/main/source/cppfront.cpp#L2712

This example shows forward can be written at call site.

cppfront/regression-tests/mixed-forwarding.cpp2

Line 21 in 38aec57

copy_from(forward t.first); // moves

and it was mentioned somewhere else.
Don't know if forward with capture makes sense.

[realgdman]

Probably related:
x := (forward T<int>());
assert on previous line.
cppfront: source/cppfront.cpp:2711: void cpp2::cppfront::emit(cpp2::postfix_expression_node &, bool): Assertion `n.expr->get_token()' failed.

[JohelEGP]

Resolving #408 might happen to fix this.

[JohelEGP]

This fails with the same error (https://cpp2.godbolt.org/z/49jcdPja6):

to: (forward v) = (forward v.to<i32>());
main: () = { }

cppfront: source/cppfront.cpp:2734: void cpp2::cppfront::emit(cpp2::postfix_expression_node&, bool): Assertion `!current_args.back().ptoken' failed.
Program terminated with signal: SIGSEGV
Compiler returned: 139

[JohelEGP]

I tried to work around this by adding parentheses.
It simply lowers forward without std::.
I suppose it's due to commit c39a94c (#77).
(forward (r$)); lowers to return forward(_0); (https://cpp2.godbolt.org/z/YTP8vd61K).

Expected result
Diagnostic, probably disallowing forward with capture

This could work with std::forward_like<decltype(this)> semantics (given 4bd0c04#commitcomment-133309652): :(forward this) (forward c$).