Compile error when returning values from a tl::expected with a void expected value

[SimonKagstrom]

Environment (from conan):

• trompeloeil v45
• tl-expected/20190710
• g++ 11.4.0, -std=gnu++20

I have a problem with mocking tl::expected return values, which has a void expected value (i.e., for a case where only the error value is interesting). I can't figure out how to write the REQUIRE_CALL for the expected value, which in real code would just do a return {}.

Example below. I've tried various other variants, but not found a working solution.

#include <tl/expected.hpp>
#include <trompeloeil.hpp>

class IInterface
{
public:
    virtual ~IInterface() = default;

    virtual tl::expected<const char *, int> Foo() noexcept = 0;
    virtual tl::expected<void, int> Bar() noexcept = 0;
};

class MockInterface : public IInterface
{
public:
    MAKE_MOCK0(Foo, (tl::expected<const char*, int>()), noexcept final);
    MAKE_MOCK0(Bar, (tl::expected<void, int>()), noexcept final);
};

int main()
{
    MockInterface mock;

    // Works
    REQUIRE_CALL(mock, Bar()).RETURN(tl::make_unexpected(1));
    mock.Bar();

    // Works
    REQUIRE_CALL(mock, Foo()).RETURN("hej");
    mock.Foo();

    // How to write this?
    REQUIRE_CALL(mock, Bar()).RETURN({});
    mock.Bar();
}

The error, after a long list of other complaints, boils down to

tl/expected.hpp:2282:26: error: no match for 'operator*' (operand type is 'const tl::expected<void, int>')
 2282 |   return x.has_value() ? *x == v : false;

[rollbear]

Wow, what an interesting find! I'm afraid it may take a while to understand exactly where and how things go wrong here. This is not obvious to me at all.

[SimonKagstrom]

I've worked around it by changing the interface to return a (slightly contrived) value in the expected case, so no worries!

[rollbear]

Current clang/libc++ main has support for C++23 std::expected, and using that instead works, so I'm leaning towards this being a bug in tl::expected. I'll investigate more, though.

https://godbolt.org/z/qv58jnEov

[rollbear]

It also works with Martin Moene's expected-lite. https://github.com/martinmoene/expected-lite/tree/master

https://godbolt.org/z/oc8eha946

[SimonKagstrom]

Thanks!

Closing this since it's looks like a bug in tl::expected. We used expected-lite a while back, but for some reason I don't remember, we replaced it with tl::expected. Might be worth reconsidering again.