MOCK_METHOD with an std::any argument doesn't compile with clang 9.0.0

[daravi]

As the title says, the compiler returns:

.../llvm-9.0.0/bin/../include/c++/v1/type_traits:2843:14: error: base class has incomplete type
...
in instantiation of template class 'testing::internal::MockSpec<std::__1::basic_string<char> (const std::__1::any &)>' requested here
     MOCK_METHOD1(doSomething, std::string(const std::any& data));
     ^

Issue only occurred when I switched from GCC to Clang.

[asoffer]

Interesting, I don't think this is strictly a Googletest issue, as I've reproduced something that I would expect to work (in both clang and gcc)

https://godbolt.org/z/V-tHr9

[asoffer]

As it turns out, this is a known bug with the standard that occurs with type traits whenever you perfect-forward (done by std::tuple) to a type-erased interface (std::any). Internally we pass around the function arguments with a tuple. We could work around this in some instances by 1-argument tuples. I'm going to look into this, but it may be more complexity than we wish to maintain.

[asoffer]

Further reference to the issue
https://cplusplus.github.io/LWG/issue3121

[asoffer]

We've looked into this a bit. If the use of std::tuple were entirely internal, we could consider a number of solutions (storing dummy arguments to pad the tuple to >1 element, or using an alternate implementation without these issues).

However, the tuples are exposed in public interfaces (e.g., the MockSpec .With method). I think the sad reality is that, the collateral damage we would cause by fixing this is just too big. I'd suggest looking to use a type other than std::any as the argument.

Sorry we can't be more helpful on this.

[alireza-hosseini]

I have the same issue with GCC as well:
error: invalid use of incomplete type ‘struct std::is_constructible<testing::Matcher<std::any>, const testing::Matcher<std::any>&>’