Deleted default constructor identified as candidate in ambiguous error - c++

Question

When attempting to delete the default constructor and provide a new one with a default parameter (and therefore still be able to default construct the object), I receive an ambiguity error from g++.
class Thing
{
public:
Thing() = delete;
Thing(int arg = 0) : arg(arg) {}
private:
int arg;
};
int main(int, char**)
{
Thing thing;
return 0;
}
Compilation error below:
$ g++ deletedConstructorTest.C -std=c++11
deletedConstructorTest.C: In function "int main(int, char**)":
deletedConstructorTest.C:12:9: error: call of overloaded "Thing()" is ambiguous
Thing thing;
^
deletedConstructorTest.C:12:9: note: candidates are:
deletedConstructorTest.C:5:5: note: Thing::Thing(int)
Thing(int arg = 0) :arg(arg) {}
^
deletedConstructorTest.C:4:5: note: Thing::Thing() <deleted>
Thing() = delete;
^
If I modify the example above by removing the "Thing() = delete;" line then it compiles fine. I understand that the compiler won't generate a default constructor since I provided my own so that line is unnecessary but I was surprised at the compilation error.
Thanks in advance!

Answer 1

All declared overloads are considered during overload resolution, even if they are deleted or inaccessible. So you've declared two default constructors, both of which are an equally good match for default-initialisation, hence the ambiguity.
In this case, simply don't declare the zero-argument default constructor at all. The one-argument constructor is usable as a default constructor, and declaring any constructor inhibits the generation of an implicit default constructor.