bad compilation with default arguments in clang 20

[rockwotj]

https://godbolt.org/z/5s4GazKEP

#include <print>
#include <numeric>
#include <vector>
#include <string>

struct foo {std::string bar;};

const std::vector<foo> foos = {{"1"}, {"2"}, {"3"}};

size_t count(const std::vector<foo>& input = foos) {
  size_t c = 0;
  for (const foo& f : input) {
    c += f.bar.size();
  }
  return c;
}

int main() {
    std::print("answer 1: {}\n", count());
    std::print("answer 2: {}\n", count({{"1"}, {"2"}}));
    std::print("answer 3: {}\n", count());
}

When compiled in clang 20 we get 3 printed 3 times, while in clang 19 we get 3, 2, 3 printed. I suspect some kind of over-aggressive optimization is assuming the input to this function is always foos.

[efriedma-quic]

Duplicate of #141114?

[rockwotj]

Looks very likely yes

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Tyler Rockwood (rockwotj)

https://godbolt.org/z/5s4GazKEP

#include <print>
#include <numeric>
#include <vector>
#include <string>

struct foo {std::string bar;};

const std::vector<foo> foos = {{"1"}, {"2"}, {"3"}};

size_t count(const std::vector<foo>& input = foos) {
  size_t c = 0;
  for (const foo& f : input) {
    c += f.bar.size();
  }
  return c;
}

int main() {
    std::print("answer 1: {}\n", count());
    std::print("answer 2: {}\n", count({{"1"}, {"2"}}));
    std::print("answer 3: {}\n", count());
}

When compiled in clang 20 we get 3 printed 3 times, while in clang 19 we get 3, 2, 3 printed. I suspect some kind of over-aggressive optimization is assuming the input to this function is always foos.

[AaronBallman]

CC @shafik