Why does clang handle exceptions for this trivial std::variant code?

Question

If we have code like this:

#include <variant>

int main(){
    using V = std::variant<int, double>;
    V a = 5;
    V b = 5.6;

    a.swap(b);
}

https://gcc.godbolt.org/z/oqGiHs

If you compile with clang, it emits code that handle exception during swap.

Why is that? Both variants are non empty and underlying types are exception safe.

Update:

Ironically this compiles without exception:

#include <variant>

template<class T>
void sw(T &a, T &b){
    auto c = a;
    a = b;
    b = c;
}

int main(){
    using V = std::variant<int, double>;

    V a = 5;
    V b = 5.6;

    sw(a, b);
}

GCC just emits code for implicit `return 0;` [in my case](https://gcc.godbolt.org/z/CABxAg). — Daniel Langr, Jun 16 '20 at 08:34
It's a bit weird since `noexcept(a.swap(b))` is true with Clang, so it should be able to "know" that nothing in `main` can actually throw. — Daniel Langr, Jun 16 '20 at 10:06

score 2 · Accepted Answer · answered Jul 04 '20 at 11:20

2

This turns out to be clang bug.

https://bugs.llvm.org/show_bug.cgi?id=46342

It appears to be fixed, but I can not find the version where is fixed.

answered Jul 04 '20 at 11:20

Nick

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Why does clang handle exceptions for this trivial std::variant code?

Update:

1 Answers1