[cfe-dev] Issue with map and unordered_map: call to deleted constructor of 'std::pair<const int, int>'

[David Blaikie]

>
> I was wondering if this is a bug in the gcc 4.5.2 standard library or a bug
>> in clang... and I'd really appreciate a work-around if anyone has one, since
>> I'm pretty much stuck at this point.
>
>
> As a baseline, this code compiles successfully when using libc++ (compile
> with -stdlib=libc++, though you'll of course need to install libc++) & I
> seem to have a repro of your errors with libstdc++ so I'm playing around
> with that now.
>

I've investigated this a bit further with some help & I believe this is a
bug in clang ToT surrounding implicit move constructor/assignment.

Given this:

struct foo {
  void operator=(foo&&);
};

int main() {
  foo g((foo()));
}


Clang's output is:

    simple.cc:6:7: error: call to deleted constructor of 'foo'
foo g((foo()));          ^ ~~~~~~~    simple.cc:1:8: note: function
has been explicitly marked deleted here    struct foo {           ^

[g++ compiles this without error]

Clang seems to be implementing not providing implicit move operations by
implementing them as deleted rather than as not present. This would be
incorrect & produces the failure above.

The reason this came up in std::pair is a bit weird - it seems libstdc++'s
std::pair has none of the big 5 (copy ctor, move ctor, copy assignment, move
assignment, destructor) /except/ the move assignment operator. It does
however have member function templates that, while not technically copy/move
construction/assignment, are probably good enough for most cases. (returning
a temporary probably should invoke the copy ctor of libstdc++'s pair,
though, since it doesn't have a real move ctor which it should have)

Anyway, the presence of the move assignment operator caused clang to provide
a deleted move ctor rather than no move ctor at all.

So there's the tale - I'll look into a fix & you'd be welcome to file a bug
(& assign it to me, though if someone else on the list really wants to take
it I don't mind), Matthieu. Thanks for bringing this up.

[side note: we should do a better job about the warning on "The Most Vexing
Parse" (see the declaration of 'g' in the above example which required the
extra () to be a variable declaration instead of a function declaration) -
like the parantheses warning I think we should have a couple of notes
suggesting the two ways to address the issue - most likely the user actually
meant to declare a variable & should insert the extra (). But alternatively
they can avoid the warning by removing the extra () after the 'foo' in the
argument list.]

- David
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