clang:折叠表达式和"表达结果未使用"警告

Question

在clang 7.0上这段代码:

template <typename ... Ts> struct S {
  unsigned gs(unsigned i) {
    unsigned r = 0;
    ((r = unsigned(sizeof(Ts)), i-- == 0) || ...);
    return r;
  }
};

int foo(unsigned i) {
  S<int, double, long, float, char> s;
  return s.gs(3);
}

导致此警告:

~/dev/ta $ ~/bin/clang++ -c -std=c++17 fold-warning.cpp 
fold-warning.cpp:5:46: warning: expression result unused [-Wunused-value]
    ((r = unsigned(sizeof(Ts)), i-- == 0) || ...);
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    ^

为什么？编译器的逻辑是什么表达式未被使用？

Gcc 7.3/8.2很满意.

Answer 1

这一行的结果:

((r = unsigned(sizeof(Ts)), i-- == 0) || ...);

是一个顺序||操作,其结果未使用,从而导致警告.

施法以void消除警告:

(void)((r = unsigned(sizeof(Ts)), i-- == 0) || ...);