c++ 11构造函数重载解析和initialiser_lists: clang++ +和g++不同意

I have a small piece of C++11 code which g++ (4.7 or 4.8) refuses to compile claiming that the call to constructor for B2 b2a(x, {P(y)}) is ambiguous. Clang++ is happy with that code, but refuses to compile B2 b2b(x, {{P(y)}}) which g++ is perfectly happy to compile!I have a small piece of C++11 code which g++ (4