`<numeric>`: Parallel `transform_reduce` incorrectly uses list-initialization

[frederick-vs-ja]

Describe the bug

The following program doesn't compile with MSVC STL due to narrowing conversion in list-initialization.

(originally discovered by mq白 (@Mq-b) in Mq-b/Loser-HomeWork@099c66a)

#include <execution>
#include <numeric>
#include <string>
#include <vector>

int main()
{
    std::vector<std::string>strVec{"Only", "for", "testing", "purpose"};
    std::size_t res = std::transform_reduce(
        std::execution::par,
        strVec.begin(), strVec.end(),
        0,
        [](std::size_t a, std::size_t b) {return a + b; },
        [](std::string s) {return s.size(); }
    ); // the value of res is 21
}

STL/stl/inc/execution

Line 4235 in f392449

_Ty _Val{_Reduce_op(_Transform_op(*_Chunk._First), _Transform_op(*++_Next))};

Per [transform.reduce], IIUC list-initialization shouldn't be used here. We should only use implicit conversion (in other words, copy-initialization).

There're some other parallel numeric algorithms direct-initializing _Ty variables from the results of the binary operations. Perhaps they are also buggy.
(Despite being contrived, it's possible to make T(bin_op(l, r)) ill-formed while T t = bin_op(l, r); being well-formed.)

Command-line test case

Godbolt link