Incorrect copy constructor while passing argument to generic lambda

Question

Consider this piece of code:

template < auto What >
constexpr auto Identity = [](auto&&...) { return What; };

struct Ban
{
  Ban() = default;
  Ban(const Ban&  ban) = delete;
  Ban(      Ban&& ban) = delete;
};

int main()
{ 
  Ban ban;
  Identity<false>(10,ban);

  return 0;
}

This fails to compile on godbolt.org with gcc-7.3 as it tries to copy the second argument of Identity. Why it should? Is this a bug in gcc?

gcc does not complain if the second argument is a temporary or when there is only one argument. It complains for only one argument when the definition of Identity is with (...) instead of (auto&&...).

Might indeed be a bug, gcc-8 and clang happily compile this. — lubgr
– lubgr, Commented Sep 27, 2018 at 8:09
Surprisingly gcc-8 fails when changing (auto&&...) to just (...)! — Vahagn
– Vahagn, Commented Sep 27, 2018 at 8:12

T.C. · Accepted Answer · 2018-09-27 08:37:42Z

3

The first half is a mis-parse of auto&&... for generic lambdas in older GCC versions: clang vs gcc - empty generic lambda variadic argument pack; Should non-capturing generic lambdas decay to function pointers?; Should non-capturing generic lambdas decay to function pointers?

The second half is expected. Passing over C-style ... variadics makes a copy and you deleted your copy constructor.

answered Sep 27, 2018 at 8:37

T.C.

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1 Comment

Yakk - Adam Nevraumont Over a year ago

As a suggestion, include the easy fix: [](auto&&...x) giving the unused pack a name.

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