Why does {} work while () doesn't in initializing an atomic object here? [duplicate]

Question

For example, code like this:

struct A { A(int); };
struct B { B(A);   };

int main()
{
    B b{{0}}; // OK
    B c({0}); // error
}

The error messages are:

f.cc: In function 'int main()':
f.cc:7:9: error: call of overloaded 'B(<brace-enclosed initializer list>)' is ambiguous
  B c({0}); // error

         ^
f.cc:7:9: note: candidates are:
f.cc:2:12: note: B::B(A)
 struct B { B(A);   };
        ^
f.cc:2:8: note: constexpr B::B(const B&)
 struct B { B(A);   };
        ^
f.cc:2:8: note: constexpr B::B(B&&)

Answer 1

As of the latest official standard, C++14, your first initialization is not ambiguous. [over.match.list]:

As no initializer-list constructors exist, we enter the "second phase". And now consider [over.best.ics]/4:

Our element is {0}. Hence this disallows the (user-defined) conversion {0} -> A for the copy constructor. Clearly, this doesn't apply if we aren't in the second phase of [over.match.list], so for your example with B c({0}), no list-initialization occurs for c and both constructors are considered.

---

CWG issue 1467

The first initialization is currently just as ambiguous as the second one. Compilers simply haven't implemented CWG #1467 yet - its resolution removed bullet point (4.5), quoted above.
See #2076, which opts to revert the change:

The resolution of issue 1467 made some plausible constructs ill-formed. For example,

struct A { A(int); };
struct B { B(A); };
B b{{0}};

This is now ambiguous, because the text disallowing user-defined conversions for B's copy and move constructors was removed from 13.3.3.1 [over.best.ics] paragraph 4.

"The text" is the aforementioned bullet point. Richard Smith proposes the following wording:

For non-class types, we allow initialization from a single-item list to perform a copy only if the element within the list is not itself a list (13.3.3.1.5 [over.ics.list] bullet 9.1). The analogous rule for this case would be to add back the bullet in 13.3.3.1 [over.best.ics] paragraph 4, but only in the case where the initializer is itself an initializer list:

        the second phase of 13.3.1.7 [over.match.list] when the initializer list has exactly one         element that is itself an initializer list, where the target is the first parameter of a constructor
        of class X, and the conversion is to X or reference to (possibly cv-qualified) X,

As the initializer {0} is itself an initializer list, that bullet point would make your first initialization well-formed again.