We're missing deduction guides for std::function. The following code does not compile with libc++ trunk:

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#include <functional>

int func(double) { return 0; }
int main() {
  std::function f{func}; // guide #1 deduces function<int(double)>
  int i = 5;
  std::function g = [&](double) { return i; }; // guide #2 deduces function<int(double)>
}
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Taken from SO question: https://stackoverflow.com/q/53217511/1708801