[BUG] std::array usage

[jmafc]

Describe the bug
cppfront -p allows both operator[] and at() on a std::array, but these result in errors in g++.

To Reproduce
Steps to reproduce the behavior:

1. Sample code

main: () = {
    arr: std::array<std::string, 5>;
    arr[0] = "some";
    arr.at(1) = "thing";
}

2. Command lines including which C++ compiler you are using

cppfront compiler v0.2.1 Build 8724:0846
g++ (Debian 12.2.0-14) 12.2.0

$ cppfront test.cpp2 -p
test.cpp2... ok (all Cpp2, passes safety checks)

$ c++ -std=c++20 -I ~/src/include test.cpp

4. Expected result

Although it's unclear whether std::array is properly supported, I was hoping that at least the .at() syntax would be accepted (and cppfront did accept both, as shown above) and subsequently compiled by g++.

5. Actual result/error

The compilation step results in the following:

In file included from test.cpp:7:
/home/jma/src/include/cpp2util.h: In instantiation of ‘decltype(auto) cpp2::assert_in_bounds(auto:56&&, auto:57&&) requires !((is_integral_v<typename std::remove_cvref<decltype(cpp2::assert_in_bounds::arg)>::type>) && requires{std::ssize(cpp2::assert_in_bounds::x);(cpp2::assert_in_bounds::x)[cpp2::assert_in_bounds::arg];std::begin(cpp2::assert_in_bounds::x) + 2;}) [with auto:56 = deferred_init<std::array<std::__cxx11::basic_string<char>, 5> >&; auto:57 = int]’:
test.cpp2:3:27:   required from here
/home/jma/src/include/cpp2util.h:411:28: error: no match for ‘operator[]’ (operand types are ‘cpp2::deferred_init<std::array<std::__cxx11::basic_string<char>, 5> >’ and ‘int’)
  411 |     return CPP2_FORWARD(x) [ CPP2_FORWARD(arg) ];

If the arr[0] line is commented out, g++ outputs the following:

In file included from test.cpp:7:
test.cpp2: In instantiation of ‘main()::<lambda(auto:89&&, auto:90&& ...)> [with auto:89 = cpp2::deferred_init<std::array<std::__cxx11::basic_string<char>, 5> >; auto:90 = {int}]’:
test.cpp2:4:5:   required from here
/home/jma/src/include/cpp2util.h:718:24: error: ‘at’ was not declared in this scope
  718 |         return FUNCNAME(CPP2_FORWARD(obj), CPP2_FORWARD(params)...); \
test.cpp2:4:5: note: in expansion of macro ‘CPP2_UFCS’
    4 |     arr.at(1) = "thing";
      |     ^~~~~~~~~

Additional context
I have been alpha-testing cppfront with some "beginners" C++-11 programs. It would be very helpful if there was a simple list of keywords/features with an indication of support status.

[JohelEGP]

You're missing the initializer, see https://cpp2.godbolt.org/z/48vhscfKj or https://cpp2.godbolt.org/z/WbGdec3bz:

-    arr: std::array<std::string, 5>;
+    arr: std::array<std::string, 5> = ();

     arr: std::array<std::string, 5>;
+    arr = ();

[JohelEGP]

You should've gotten an error like https://cpp2.godbolt.org/z/YraaP1oTK:

main.cpp2(3,9): error: local variable arr is used before it was initialized
  ==> program violates initialization safety guarantee - see previous errors

So there's definitely a bug in Cppfront.

[jmafc]

Ok, yes, I just discovered that when I tried to call a cpp1 function to add the string. Then it did tell me that I was trying to use the array before initializing it.

[jmafc]

One other thing that I've noticed (although this may be unrelated to the bug described above), is that if cppfront is invoked without the -p, the generated .cpp does not include the line

#define CPP2_USE_MODULES         Yes

and as a result, g++ then fails with the message that std::array ... has initializer but incomplete type.

[JohelEGP]

Yeah.
-pure-cpp2 implies import std;.
So without -p, "cpp2util.h" happens to not #include <array>, so you need that.

[hsutter]

Thanks. Good points... I'm working on a commit to give a better diagnostic for the lack of first use, and to always emit a deferred constructions as list initialization (which I'd already been moving towards in recent commits... sorry, vector<size_t>::vector(size_t,size_t)).

So this example:

arr: std::array<std::string, 5>;
arr[0] = "some";
arr.at(1) = "thing";

will get the nicer error error: local variable arr is used before it was initialized.

And this fixed example:

arr: std::array<std::string, 5>;
arr = ("a","b","c","d","e");  // now fixed
arr[0] = "some";
arr.at(1) = "thing";

will now work.