[llvm-bugs] [Bug 48587] New: Clang incorrectly discards destructor if there is only a constant evaluated branch

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48587

            Bug ID: 48587
           Summary: Clang incorrectly discards destructor if there is only
                    a constant evaluated branch
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mschellenbergercosta at googlemail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

This is an issue that came up with the implementation of constexpr vector and
constexpr string for MSVCs standard library.

The regression is that the destructor of iterators is not executed properly,
which lets the iterator debug machinery of the STL crash. 

A minimal reproducer of he issue is the following code:

```
struct conditionalDestructor {
    constexpr conditionalDestructor(int& val_) noexcept : val(val_) {}
    constexpr ~conditionalDestructor() noexcept {
        if (!__builtin_is_constant_evaluated()) {
            ++val;
        }
    }

    int& val;
};

int main() {
    int destructorsCalled = 0;
    {
        [[maybe_unused]] volatile conditionalDestructor
test(destructorsCalled);
    }
    if (destructorsCalled != 1) {
        return -1;
    }
}
```

Here, the destructor of `conditionalDestructor` is never executed, despite it
being runtime only.

Note that if there is code in the else branch of the
`__builtin_is_constant_evaluated` clause, the destructor is not discarded.

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