[BUG] Returning a tuple with unnamed members

[ntrel]

Possibly this should be an enhancement request. Some of these errors may be because the feature is not implemented? Perhaps I'm using the wrong syntax? If so, better diagnostics would be nice, especially for f (see 4).

To Reproduce

d: () -> (int, int) = (1,2);
e: () -> (int, int) = { return (1,2); }
f: () -> _ = (5, "hi");
g: () -> (int, std::string) = (5, "hi");

main: () -> int = {
    auto [a,b] = f();
    assert(a == 5);
    assert(b == "hi");
}

1. d can't be initialized by an expression:

rettup.cpp2(3,1): error: a function with named return value(s) must have a full { } body

But d doesn't used named return values. Also f can be initialized by a tuple expression without needing {}.

2. Commenting out d, spurious compile error for e:

rettup.cpp2(1,16): error: int - variable must be initialized on every branch path

There is no declared variable.

3. Commenting out e, spurious compile error for g:

rettup.cpp2(3,19): error: expected , in parameter list (at '::')
rettup.cpp2(3,19): error: missing function return after -> (at '::')

Probably it's parsing std as an identifier.

4. Commenting out g, f compiles OK but doesn't declare or use a tuple struct so the c++ compiler errors:

#line 2 "old/rettup.cpp2"
[[nodiscard]] auto f() -> auto { return 5, "hi";  }

#line 5 "old/rettup.cpp2"
[[nodiscard]] auto main() -> int{
    cpp2::assert_in_bounds(auto, a, b) = f();

[JohelEGP]

https://github.com/hsutter/cppfront/blob/main/regression-tests/mixed-multiple-return-values.cpp2 doesn't demonstrate it, but f: () -> (i: int, s: std::string) returns a structure with data members i and s, just like in the parameter list.

[ntrel]

@JohelEGP Yes, this is about unnamed members. BTW is there a pure cpp2 way to write auto [a,b] = f();?

[JohelEGP]

@JohelEGP Yes, this is about unnamed members.

Ah, I see. IIRC, you're right here.

Possibly this should be an enhancement request.

BTW is there a pure cpp2 way to write auto [a,b] = f();?

Not yet, it seems.

[filipsajdak]

A couple of issues here.

1. auto [a,b] = f(); is not a valid cpp2 syntax - the cpp2 rule is when you declare something there need to be : (colon) after the name - I assume it can look like the following: [a,b]:=f();. Also, structured binding still needs to be supported.

2. you need to provide the type that will be returned (if there is no -> _ or -> type the return void is assumed): f: () -> _ = (5, "hi");. You use generic type _ but you did not provide the type on the return side so compiler cannot deduce it, and that is an easiest fix e.g.:

f: () -> _ = std::tuple(5, "hi");

main: () -> int = {
    t := f();
    assert(t.get<0>() == 5);
    assert(t.get<1>() == std::string("hi"));
}

Generates:

[[nodiscard]] auto f() -> auto { return std::tuple(5, "hi");  }

[[nodiscard]] auto main() -> int{
    auto t {f()}; 
    assert(CPP2_UFCS_TEMPLATE_0(get, (<0>), t)==5);
    assert(CPP2_UFCS_TEMPLATE_0(get, (<1>), std::move(t))==std::string("hi"));
}

3. When you are using multivalue return you need to provide name and you need to initialize named return variables:

f: () -> (a:int, b:int) = { a = 5; b = 7; }

main: () -> int = {
    t := f();
    assert(t.a == 5);
    assert(t.b == 7);
}

Generates:

[[nodiscard]] auto f() -> f__ret{
                                cpp2::deferred_init<int> a;
                                cpp2::deferred_init<int> b;
#line 1 "../tests/tuple_return.cpp2"
a.construct(5); b.construct(7);
return  { std::move(a.value()), std::move(b.value()) }; }
[[nodiscard]] auto main() -> int{
    auto t {f()}; 
    assert(t.a==5);
    assert(std::move(t).b==7);
}

[ntrel]

auto [a,b] = f(); is not a valid cpp2 syntax

Mixed C++ and C++2 is OK though AFAIU, it's done in regression-tests/mixed-multiple-return-values.cpp2.

I assume it can look like the following: [a,b]:=f();

Or (a, b) := f(); perhaps, for consistency with tuple syntax.

you need to provide the type that will be returned

OK, though it would be good to error when trying to infer the type of e.g. (5, "hi") if it's not allowed there. Also:

f: () -> _ = std::tuple(5, "hi");
main: () -> int = {
    auto [a,b] = f();
}

5. The destructuring in main doesn't work, it generates:

    cpp2::assert_in_bounds(auto, a, b) = f();

When you are using multivalue return you need to provide name

OK, but if we have tuple types, it would be nice if you could use a tuple type as the return type and not have to declare member names.

[JohelEGP]

OK, but if we have tuple types, it would be nice if you could use a tuple type as the return type and not have to declare member names.

That's why I replied:

@JohelEGP Yes, this is about unnamed members.

Ah, I see. IIRC, you're right here.

Possibly this should be an enhancement request.

IOW, this is a feature request.

It's not the problem solved by this feature, but sounds like a reasonable extension.

The destructuring in main doesn't work, it generates:

Cpp2 is distinguished from C++1 at the top-level declarations. Once you're in a C++1 top-level declaration, you stay in C++1. Same for Cpp2.

namespace x {
  // Only C++1 here.
}
x: namespace = {
  // Only Cpp2 here.
}

[filipsajdak]

OK, but if we have tuple types, it would be nice if you could use a tuple type as the return type and not have to declare member names.

@ntrel you can return a tuple type:

f1: () -> _ = std::tuple(5, "hi"); // using deduced return type

f2: () -> std::tuple<int, std::string> = std::tuple(5, "hi"); // using explicit return type

[ntrel]

Cpp2 is distinguished from C++1 at the top-level declarations. Once you're in a C++1 top-level declaration, you stay in C++1. Same for Cpp2.

OK, thanks. So deducing an expression constructing a std::tuple does work but the destructuring has to be done in a C++1 declaration.

I think these should be diagnosed (better) in a C++2 function:

f: () = {
    ...
    auto x = 1; // causes ill-formed initializer error but line is first line of `f`, not this line
    auto [x,y] = (1,2); // passes cppfront but errors in C++1
}

[ntrel]

I think these should be diagnosed (better) in a C++2 function:

Implemented in #342.

[orent]

@JohelEGP Yes, this is about unnamed members. BTW is there a pure cpp2 way to write auto [a,b] = f();?

IIUC, comma expressions are gone from cpp2 so it could be just

a, b := f();

Can we have general destructuring assignment to any existing lvalues without necessarily defining new variables?

[filipsajdak]

@orent Regarding using comma:

(Have I mentioned Cpp2 has no comma operator? 😁 Or at least it's trying very hard not to have one, even accidentally.)

Originally posted by @hsutter in #331 (comment)

[ntrel]

Triage:

d: () -> (int, int) = (1,2);
e: () -> (int, int) = { return (1,2); }

1. d can't be initialized by an expression:

The error should be that an identifier was expected, not int. I have a fix for that.
If this was to be supported the syntax would probably be d: () -> (:int, :int) = .

g: () -> (int, std::string) = (5, "hi");
rettup.cpp2(3,19): error: expected , in parameter list (at '::')

I think for that parameter the error is correct as std can be an identifier and the parser expects an unqualified identifier.

f: () -> _ = (5, "hi");
Commenting out g, f compiles OK but doesn't declare or use a tuple struct so the c++ compiler errors:

Type inference + initializer list could be diagnosed with an error.

There is also another bug:

b: () -> (a, b:_) = (1,2);

This should error that no type is declared for a and b. I have a fix for that.