Strange derived allocator behaviour

[Bonita Montero]

Consider the following code:

#include <vector>
#include <memory>

using namespace std;

template<typename Alloc>
void f( Alloc const &alloc )
{
using IAlloc = typename allocator_traits<Alloc>::template
rebind_alloc<int>;
vector<int, IAlloc> viA( IAlloc( alloc ) ); // doesn't work
vector<int, IAlloc> viB( IAlloc( const_cast<Alloc const &>(alloc) ) );
};


int main()
{
f( allocator<char>() );
}

Why does the first vector-definition not work (at least with MSVC and
clang 11).

[Bonita Montero]

g++, even an older version, does compile it.

[Bonita Montero]

> Why does the first vector-definition not work (at least with MSVC and
> clang 11).

Ok, found it myself: it seems to be a function-definition.
But why is it parsed as a function-definition although IAlloc( alloc )
isn't a valid parameter-definition ?

[Paavo Helde]

IAlloc(alloc) is parsed as 'IAlloc alloc' and is a valid parameter
definition. Here is a simplified version of your line:

vector<int, IAlloc> viA(int(alloc));

which is also parsed as a function declaration:

vector<int, IAlloc> viA(int alloc);

To get it parsed as a variable definition add some parens:

vector<int, IAlloc> viA((IAlloc(alloc))); // does work

I agree these parsing rules are obnoxious and should be changed somehow.