gcc警告为何统一初始化导致缩小转换，其背后原理究竟为何？

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我尝试将长类型变量转换为具有+uniform+initialization的int类型变量，但没有成功。我只能通过统一初始化获得编译器警告。这是为什么？为什么gcc不报告两种情况？我也尝试过使用clang，并得到了类似的结果。

我试图将长类型变量转换为具有 uniform initialization的int类型变量,没有它.但是我只能通过统一初始化获得编译器警告.这是为什么？为什么gcc不警告两种情况？我也尝试过clang并得到了类似的结果.

这是代码

#include <iostream> int main() { long l = 1; int i1 = l; int i2 = { l }; std::cout << i1 << std::endl; std::cout << i2 << std::endl; return 0; }

我得到的唯一一个警告

$g++ -Wall -Wextra 1.cpp 1.cpp: In function ‘int main()’: 1.cpp:6:16: warning: narrowing conversion of ‘l’ from ‘long int’ to ‘int’ inside { } [-Wnarrowing] int i2 = { l }; 因为标准说, narrowing conversions limit仅指定列表初始化(自C 11起).

list-initialization limits the allowed implicit conversions by
prohibiting the following:

• conversion from a floating-point type to an integer type
• conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression
  and overflow does not occur
• conversion from an integer type to a floating-point type, except where the source is a constant expression whose value can be stored
  exactly in the target type
• conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source
  is a constant expression whose value can be stored exactly in the
  target type

对于其他初始化方法(使用括号或等号),不应用(添加)缩小转换限制规则;因为这可能会破坏很多遗留代码.