GCC 7 C++ 17 support for folding expressions

Question

The following snippet will compile in GCC 8+, but fails to compile in GCC 7.

template <typename... THINGS>
struct A
{
  explicit A(THINGS *... things)
  {
    (..., [thing = things](){}());
  }
};


int main()
{
    int thing;
    const auto thingy = A{&thing};
}

godbolt

The stated failure is that the parameter pack isn't being expanded: parameter packs not expanded with '...'.
Checking the GCC standards compliance page, fold expressions should be supported in GCC 7.

Is there another flag i need besides std=c++17? (i didn't see one)
Is the standard not yet completely implemented? (i didn't see anything indicating that)
Can i make this work, or is this just a GCC 7 bug i'm going to have to work around?

FWIW you should be able to use the array trick: `int discard[] = {([thing = things](){}(), 0)...};` — NathanOliver, Oct 24 '19 at 21:15
@NathanOliver But this gives the same error, because of the same GCC bug? — walnut, Oct 24 '19 at 21:17
@uneven_mark Oh. I thought the issue was the fold expression itself, not that the lambda capture was breaking the expression. Now that I've actually read the bug I see that it will indeed not work. — NathanOliver, Oct 24 '19 at 21:20

score 6 · Accepted Answer · answered Oct 24 '19 at 21:16

This is a GCC bug, originally reported in version 8.01, fixed in version 8.2. It seems that the bug also occurs when fold-expressions are not used (the C++11-era "expander trick" mentioned by NathanOliver doesn't work either), so you'll have to use a longer workaround that doesn't require expanding a template parameter pack that is inside a lambda capture. For example:

template <typename THING>
void do_it(THING* thing) {
    [thing]{}();
}

explicit A(THINGS *... things)
{
    (..., do_it(things));
}

score 4 · Answer 2 · answered Oct 24 '19 at 21:13

4

This is a GCC bug. Found the tracked issue, fixed in 8.2.

answered Oct 24 '19 at 21:13

eerorika

232,697
12
197
326

GCC 7 C++ 17 support for folding expressions

2 Answers2