带 auto 的 initializer_list 包含多个表达式

Question

Rather simple question,

auto x11 {1,2,3,4};
auto x1 = {1,2,3,4};
auto x22 {1.0, 2.25, 3.5};
auto x2 = {1.0, 2.25, 3.5};

As far as I understand, there should be no difference here with respect to having = or not. However, using llvm/clang 6.0.0 (with --std=c++17), I get :

main1.cpp:35:17: error: initializer for variable 'x11' with type 'auto' contains multiple
  expressions
auto x11 {1,2,3,4};
~~~~~~~~    ^

main1.cpp:37:20: error: initializer for variable 'x22' with type 'auto' contains multiple
  expressions
auto x22 {1.0, 2.25, 3.5};

From Stroustroup's C++ book, page.162:

auto x1 {1,2,3,4}; // x1 is an initializer_list<int>
auto x2 {1.0, 2.25, 3.5 }; // x2 is an initializer_list of<double>

So, is there really a problem in not having = in there?

Answer 1

The rule of auto type deduction changed since N3922 . (This is considered as a defect in C++14).

In direct-list-initialization (but not in copy-list-initalization), when deducing the meaning of the auto from a braced-init-list, the braced-init-list must contain only one element, and the type of auto will be the type of that element:

 auto x1 = {3}; // x1 is std::initializer_list<int> auto x2{1, 2}; // error: not a single element auto x3{3}; // x3 is int // (before N3922 x2 and x3 were both std::initializer_list<int>)

So before N3922, all the variables in your sample work fine and have type std::initializer_list<int> . But since N3922, for direct initialization (ie for x11 and x22 ) the braced-initializer must contain only one element (and their type would be the type of the element), then the code become ill-formed.

See N3922 and N3681 for more.