匿名临时对象和 class 模板参数推导 - gcc vs clang

Question

考虑以下代码片段：

template <typename T>
struct foo
{
    foo(T) { }
};

int main()
{
    foo{0};
}

g++ 7愉快地创建了一个foo类型的临时对象，推导T = int.

clang++ 5和6拒绝编译代码：

error: expected unqualified-id
    foo{0};
       ^

live example on wandbox

这是一个 clang 错误，还是标准中有什么东西阻止 class 模板参数推导 为未命名的临时对象启动？

Answer 1

Clang 错误 (#34091)

来自[dcl.type.class.deduct]：

A placeholder for a deduced class type can also be used in [...] or as the simple-type-specifier in an explicit type conversion (functional notation). A placeholder for a deduced class type shall not appear in any other context. [ Example:
template<class T> struct container {
    container(T t) {}
    template<class Iter> container(Iter beg, Iter end);
};

template<class Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;
std::vector<double> v = { /* ... */ };

container c(7);                         // OK, deduces int for T
auto d = container(v.begin(), v.end()); // OK, deduces double for T
container e{5, 6};                      // error, int is not an iterator
— end example ]

匿名临时对象和 class 模板参数推导 - gcc vs clang

Anonymous temporaries and class template argument deduction - gcc vs clang

c++

language-lawyer

class-template

template-argument-deduction

c++17