[cxx-interop] Fix test header to declare linkage for a implicit template

[drodriguez]

The header was defining a function, the function created a lambda, and the lambda was transformed into a std::function. This transformation is incorrect because the function scope does not have linkage, so the instantiated types will not have linkage either, causing the error below.

.../include/c++/11.2.0/bits/invoke.h:104:5:
error: function 'std::__invoke_r<int, (lambda at .../swift/test/Interop/Cxx/stdlib/Inputs/std-function.h:9:10) &, int>' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
102 │   template<typename _Res, typename _Callable, typename... _Args>
103 │     constexpr enable_if_t<is_invocable_r_v<_Res, _Callable,
    _Args...>, _Res>
    104 │     __invoke_r(_Callable&& __fn, _Args&&... __args)
        │     ╰─ error: function 'std::__invoke_r<int, (lambda at .../swift/test/Interop/Cxx/stdlib/Inputs/std-function.h:9:10) &, int>' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
        105 │     noexcept(is_nothrow_invocable_r_v<_Res, _Callable,
            _Args...>)
  106 │     {

  }

Declaring the function inline forces each TU to have their own copies of the function, which avoids the instantiated templates from being in a different TU than the one using them.

The header would not have worked in a normal C++ program, since none of the functions declare linkage, they would have been defined in each TU that included the header and it would fail linking as soon as two of those TU tried to be linked together. Declaring the functions inline avoids the problem (a more normal header, only with declarations, would also worked).

[drodriguez]

@egorzhdan this is what I was talking about.

I don't think this is because any changes in our headers, but I don't understand why you are not seeing it on your side.

If you try to use that header in a C++ program you would had hit some of those issues as well. They are simply hidden by the fact that only one TU is being used in the test.

[drodriguez]

@swift-ci please test

[egorzhdan]

Oh, interesting! Thank you for investigating this.

[egorzhdan]

@swift-ci please test