<algorithm>: Silent bad codegen for vectorized meow_element() above 4 GB

[StephanTLavavej]

Reported by Johan Lundberg and Felix Ungman. This is a regression in VS 2022 17.4 after the PR series #2447 #2821 #2825.

This repro is specific to x64 because it requires a range over 4 gigabytes. The size in bytes, not the size in elements, is the threshold, so we can repro it with slightly over 1 billion ints.

D:\GitHub\STL\out\x64>type meow.cpp

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main() {
    for (const auto n : {0x3FFF'FFFFull, 0x4000'0010ull}) {
        vector<int> v(n, 25);
        v[n - 2] = 24;
        v[n - 1] = 26;

        cout << "n: " << n << "\n";
        cout << "min_element: " << *min_element(v.begin(), v.end()) << "\n";
        cout << "max_element: " << *max_element(v.begin(), v.end()) << "\n";
        const auto p = minmax_element(v.begin(), v.end());
        cout << "minmax_element: " << *p.first << ", " << *p.second << "\n";
        cout << "\n";
    }
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MT /O2 meow.cpp && meow
meow.cpp
n: 1073741823
min_element: 24
max_element: 26
minmax_element: 24, 26

n: 1073741840
min_element: 25
max_element: 25
minmax_element: 25, 25

The escape hatch restores correctness:

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MT /O2 /D_USE_STD_VECTOR_ALGORITHMS=0 meow.cpp && meow
meow.cpp
n: 1073741823
min_element: 24
max_element: 26
minmax_element: 24, 26

n: 1073741840
min_element: 24
max_element: 26
minmax_element: 24, 26

I found the root cause and am working on a fix. The code isn't ignoring data beyond 4 GB - it actually finds the right answers, but then has a 32-bit truncation mistake immediately before returning the result.

I verified that no other vectorized algorithms are affected, as they don't perform these advanced index computations.