c++ - 在折叠表达式中扩展参数包作为 lambda 捕获的一部分 - gcc 与 clang

Question

考虑以下代码片段:

template <int... Is>
void foo()
{
    ([i = Is]{}(), ...); 
}

• clang++ (trunk) 使用 -std=c++17

成功编译代码

• g++ (trunk) 编译失败，错误如下:

  <source>: In function 'void foo()':
  
  <source>:4:11: error: parameter packs not expanded with '...':
      ([i = Is]{}(), ...);
          ^~
  
  <source>:4:11: note:         'Is'
  <source>:4:16: error: operand of fold expression has no unexpanded parameter packs
      ([i = Is]{}(), ...);
      ~~~~~~~~~~^~
  

  on godbolt.org

这是一个 g++ 错误，还是标准阻止将参数包扩展为 lambda-introducer 的一部分？

Answer 1

这看起来像是一个错误。

[temp.variadic]/4

A pack expansion consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list (described below). The form of the pattern depends on the context in which the expansion occurs. Pack expansions can occur in the following contexts:

• ... [not relevant]
• In a fold-expression; the pattern is the cast-expression that contains an unexpanded parameter pack.

带有函数调用的完整 lambda 表达式(如您所见)是有效的 cast-expression如果有人遵循语法产生式。没有理由将其排除在有效模式之外。