Overload resolution failure with multi-element initializer_list #38928
Description
| Bugzilla Link | 39580 |
| Version | 7.0 |
| OS | All |
| CC | @DougGregor,@mclow,@zygoloid |
Extended Description
Test case:
#include <iostream>
#include <string>
#include <vector>
namespace {
struct Attribute
{
explicit Attribute(std::string name, std::string value)
: name(name), value(value) {}
std::string name;
std::string value;
};
struct Print
{
void print(const std::string& s)
{ std::cout << "[String] " << s << std::endl; }
void print(const std::vector<Attribute>& v)
{ std::cout << "[Vector] " << v[0].name << std::endl; }
};
}
int main()
{
const Attribute foo("foo", "True");
const Attribute bar("bar", "True");
Print p;
p.print( {foo} );
p.print( {foo, bar} );
return 0;
}Compiling this with Clang 7 (or 6 for that matter), using "-std=c++17 -stdlib=libc++" produces:
error: call to member function 'print' is ambiguous
p.print( { foo, bar } );
note: candidate function
void print(const std::string& s)
note: candidate function
void print(const std::vector<Attribute>& v)
I am not sure whether this is a Clang bug or a libc++ bug. Switching to libstdc++ makes it compile and run just fine. And you get the same error using g++ with libc++ as well, so it's at least in some ways related to the library implementation.
AFAIU, the call with the single-element initializer_list could hit std::string's copy constructor (which might make this related to bug 23812) and therefore discard that overload. That would leave std::vector's initializer_list constructor as the only viable option (I believe), and therefore it works.
But the failing multi-element initializer_list call... It's not telling me which std::string constructor it is considering, so I find it hard to speculate further what's going on there.
There's several ways to work around the problem. Using double braces works, presumably as that reduces it to a single-element initializer_list again. Going via a initializer_list variable also works; auto li = { foo, bar }; t.test(li);