[BUG]

[iprtel]

Describe the bug
cppfront produces CPP1 code that is no accepted by gcc-11.4 or clang-18.0.0

To Reproduce

1. in issue_base.h

class IssueBase {
public:
    virtual bool virtualConstFunction() const = 0;
    virtual bool virtualFunction() = 0;
};

2. in issue.cpp2

#include "issue_base.h"
Issue : type = {
    virtualConstFunction : (this) -> bool = {
        std::cout << "virtual const function called\n";
        return true;
    }
    virtualFunction : (inout this) -> bool = {
        std::cout << "virtual function called\n";
        return false;
    }
}

main : () = {
    issue : Issue = ();
    issue.virtualFunction();
    issue.virtualConstFunction();
    issue.virtualFunction();
}

Cppfront generates std::move(issue) for the last use of issue in main. This works fine if virtualConstFunction is the last function called but fails if virtualFunction is the last call.

gcc-11.4.0:

[proc] Executing command: /usr/bin/cmake --build /workspaces/cpp2_learn/build --config Debug --target issue -j 10 --
[build] [1/3 33% :: 0.007] Generating ../_cppfront/issue.cpp
[build] /workspaces/cpp2_learn/issue/issue.cpp2... ok (mixed Cpp1/Cpp2, Cpp2 code passes safety checks)
[build]
[build] [2/3 66% :: 1.423] Building CXX object issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o
[build] FAILED: issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o
[build] /usr/bin/g++ -I/workspaces/cpp2_learn/issue -isystem /workspaces/cpp2_learn/cppfront/cppfront/include -g -std=gnu++20 -MD -MT issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o -MF issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o.d -o issue/CMakeFiles/issue.dir/__/_cppfront/issue.cpp.o -c /workspaces/cpp2_learn/build/_cppfront/issue.cpp
[build] In file included from /workspaces/cpp2_learn/build/_cppfront/issue.cpp:7:
[build] /workspaces/cpp2_learn/issue/issue.cpp2: In instantiation of 'main()::<lambda(auto:80&&)> [with auto:80 = Issue]':
[build] /workspaces/cpp2_learn/issue/issue.cpp2:19:5: required from here
[build] /workspaces/cpp2_learn/issue/issue.cpp2:19:5: error: 'virtualFunction' was not declared in this scope
[build] 19 | issue.virtualFunction();
[build] | ^
[build] ninja: build stopped: subcommand failed.

clang-18.0.0:

[build] [1/3 33% :: 0.006] Generating ../_cppfront/issue.cpp
[build] /workspaces/cpp2_learn/issue/issue.cpp2... ok (mixed Cpp1/Cpp2, Cpp2 code passes safety checks)
[build]
[build] [2/3 66% :: 1.704] Building CXX object issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o
[build] FAILED: issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o
[build] /usr/bin/clang++-18 -I/workspaces/cpp2_learn/issue -isystem /workspaces/cpp2_learn/cppfront/cppfront/include -g -std=gnu++20 -MD -MT issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o -MF issue/CMakeFiles/issue.dir//_cppfront/issue.cpp.o.d -o issue/CMakeFiles/issue.dir/__/_cppfront/issue.cpp.o -c /workspaces/cpp2_learn/build/_cppfront/issue.cpp
[build] /workspaces/cpp2_learn/issue/issue.cpp2:19:17: error: use of undeclared identifier 'virtualFunction'
[build] 19 | CPP2_UFCS_0(virtualFunction, std::move(issue));
[build] | ^
[build] /workspaces/cpp2_learn/issue/issue.cpp2:19:5: note: in instantiation of function template specialization 'main()::(anonymous class)::operator()' requested here
[build] 19 | CPP2_UFCS_0(virtualFunction, std::move(issue));
[build] | ^
[build] /workspaces/cpp2_learn/cppfront/cppfront/include/cpp2util.h:797:2: note: expanded from macro 'CPP2_UFCS_0'
[build] 797 | }(PARAM1)
[build] | ^
[build] 1 error generated.
[build] ninja: build stopped: subcommand failed.

The generated main:

#line 14 "/workspaces/cpp2_learn/issue/issue.cpp2"
auto main() -> int{
    Issue issue {}; 

    CPP2_UFCS_0(virtualFunction, issue);
    CPP2_UFCS_0(virtualConstFunction, issue);
    CPP2_UFCS_0(virtualFunction, std::move(issue));
}

If the std::move(...) is removed around issue both compilers accept the code.

[JohelEGP]

For the implicit move, see #305 (comment).
You need to add _ = issue;.

To actually declare Issue as an IssueBase,
you need a this: IssueBase; data member.
To override the virtual functions,
you need override this and override inout this parameters.

[iprtel]

Thanks. So my simplified test case is issue.cpp2. I still get the same error.

Issue : type = {
    constFunction : (this) -> bool = {
        std::cout << "const function called\n";
        return true;
    }
    nonConstFunction : (inout this) -> bool = {
        std::cout << "non const function called\n";
        return false;
    }
}

main : () = {
    issue : Issue = ();
    _ = issue.nonConstFunction();
    _ = issue.constFunction();
    _ = issue.nonConstFunction();
}

Thanks for the help on the inheritance. I struggled with that.

[JohelEGP]

You need to discard the last use of issue, without any calls.

You need to add _ = issue;.

[iprtel]

So if the last function you called is non-const you need to discard the object.
Thanks.

[iprtel]

@JohelEGP sorry to come once again.
My understanding about the discarding after the last non-const call is not sufficient.
This mainalso fixes the problem:

main : () = {
    issue : Issue = ();
    _ = issue.nonConstFunction();
    _ = issue.constFunction();
    result : _ = issue.nonConstFunction();
    std::cout << result ;
}

Here the move is on result :

#line 12 "/workspaces/cpp2_learn/issue/issue.cpp2"
auto main() -> int{
    Issue issue {}; 
    static_cast<void>(CPP2_UFCS_0(nonConstFunction, issue));
    static_cast<void>(CPP2_UFCS_0(constFunction, issue));
    auto result {CPP2_UFCS_0(nonConstFunction, issue)}; 
    std::cout << std::move(result);
}

[JohelEGP]

Rvalues are convertible to reference to const,
so printing << works because the rhs is taken by reference to const.

[iprtel]

So why is the discard on issue no longer needed? I thought that it was necessary since the last call had inout this and so the object counts as return value that needs to be discarded.

[JohelEGP]

operator<< typically looks like operator<<: (inout s: std::ostream, in x: T),
so x accepts moved objects.

[iprtel]

Thanks for the explenation. But I'm more interessted why

main : () = {
    issue : Issue = ();
    result : _ = issue.nonConstFunction();
}

is fine. While

main : () = {
    issue : Issue = ();
    _ = issue.nonConstFunction();
}

requires a _ = issue;

[JohelEGP]

That must be #749.

[hsutter]

You're right, both should require _ = issue;. Will fix.

The issue is that I was trying out suppressing move-from-last-use if the last use was in an initializer, because of examples like view: std::span = local_variable; when that was the last use of local_variable, which is an error for an rvalue. But that also suppresses a righteous move in examples like result := local_variable;. So I now agree those should be treated as rvalues too, and if that causes an error for an rvalue because there is no further us of that variable in scope, then the programmer must write an explicit discard of the variable.

[iprtel]

Are you at all concerned that people will just put explicit discards all over the place when trying to fix a compilation problem that the don't understand fully?