The following fails to compile on all MSVC, Clang and GCC:

struct SomeFoo;

SomeFoo myfuncReturningFoo() { return SomeFoo(); }

struct SomeFoo {};



int main()
{
    SomeFoo ffoo = myfuncReturningFoo();
}

The error message is “return type ‘struct SomeFoo’ is incomplete”.

Now this:

struct SomeFoo;

template <typename convenient_dummy = char>
SomeFoo myfuncReturningFoo() { return SomeFoo(); }

struct SomeFoo {};



int main()
{
    SomeFoo ffoo = myfuncReturningFoo();
}

Now this will compile on my MSVC. On further investigation it compiles on MSVC on Goldbolt if the -Wextra is not set (IS NOT SET), removing the -Wextra flag makes it compile. On Clang you can remove the -Wextra flag but it still doesn’t compile. Same as GCC. Removing the -Wextra flag makes it compile in the template version on MSVC.

What is making this happen?