clang: Fold expression and “expression result unused” warning

Question

On clang 7.0 this code:

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);
}

causes this warning:

~/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) || ...);
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    ^

Why? What expression is unused, by the compiler's logic?

Gcc 7.3/8.2 is happy with it.

Answer 1

The result of this line:

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

is a sequential || operation, whose result is not used, which caused the warning.

Casting to void to eliminate the warning:

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