Narrowing conversion in pair with braced initializer compiling if second attribute is not braced-initialized

Question

Since c++11, narrowing conversion is not allowed in list initialization (including aggregate initialization). So basically:

char c{1000}; // Does not compile with g++, clang, vc

But:

std::pair<char, double> p{1000, 1.0};

Compiles with all compiler?

But:

std::pair<char, double> p{1000, {1.0}};

Does not compile with VC (error C2398), gives a warning with clang and compiles silently with g++...

I would have expected VC behavior everywhere, i.e. a non-allowed narrowing conversion throwing an error. Which is compiler is right?

---

On the other hand, none of the variable declarations in the following snippets compile:

struct X {
    char c;
    double d;
};

X x1{999, 1.0};
X x2{999, {1.0}};

struct Y {
    char c;
    double d;
    Y (char c, double d) : c(c), d(d) { }
};

Y y1{999, 1.0};
Y y2{999, {1.0}};

So one of my guess may be that there is something special about std::pair? Something that would also narrowing braced-initialization?

Answer 1

1. std::pair<char, double> p{1000, 1.0}; is not diagnosed because it calls the template<class U1, class U2> pair(U1&&, U2&&) constructor (with U1 == int and U2 == double), which is an exact match; the narrowing doesn't happen until you get into the constructor body.

2. std::pair<char, double> p{1000, {1.0}}; cannot call that constructor because the braced-init-list {1.0} is a non-deduced context, so you cannot deduce U2.

   • In clang/libc++, it calls the pair(const T1&, const T2&) constructor; however clang apparently doesn't consider the conversion needed to create a temporary for reference binding as part of the narrowing check. This is probably a bug.
   • GCC/libstdc++'s pair has a template<class U1> pair(U1&&, const T2&) constructor template, which is a better match. With this constructor, the second argument isn't narrowing, and the first argument is an exact match, so you get no error or warning.