[libcxx-commits] [libcxx] [libc++] Avoid type-punning between __value_type and pair (PR #134819)

[Jorge Gorbe Moya via libcxx-commits]

slackito wrote:

The error referenced by @cmtice above should be reproducible with something like:
```
#include <set>
#include <utility>

void f() {
  std::set<std::pair<int, int>> s1;
  std::set<std::pair<int, int>> s2;

  s2 = s1;
}
```
The error message looks like this:
```
In file included from experimental/users/jgorbe/libcxx-repro/a.cc:1:
In file included from third_party/stl/cxx17/set:4:
In file included from third_party/crosstool/v18/llvm_unstable/src/libcxx/include/set:537:
third_party/crosstool/v18/llvm_unstable/src/libcxx/include/__tree:1292:5: error: const_cast from 'int' to '__key_type &' (aka 'std::pair<int, int> &') is not allowed
 1292 |     const_cast<__key_type&>(__lhs.first) = const_cast<__copy_cvref_t<_From, __key_type>&&>(__rhs.first);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
third_party/crosstool/v18/llvm_unstable/src/libcxx/include/__tree:1441:7: note: in instantiation of function template specialization 'std::__tree<std::pair<int, int>, std::less<std::pair<int, int>>, std::allocator<std::pair<int, int>>>::__assign_value<const std::pair<int, int> &, 0>' requested here
 1441 |       __assign_value(__cache.__get()->__value_, *__first);
      |       ^
third_party/crosstool/v18/llvm_unstable/src/libcxx/include/__tree:1404:5: note: in instantiation of function template specialization 'std::__tree<std::pair<int, int>, std::less<std::pair<int, int>>, std::allocator<std::pair<int, int>>>::__assign_multi<std::__tree_const_iterator<std::pair<int, int>, std::__tree_node<std::pair<int, int>, void *> *, long>>' requested here
 1404 |     __assign_multi(__t.begin(), __t.end());
      |     ^
third_party/crosstool/v18/llvm_unstable/src/libcxx/include/set:666:13: note: in instantiation of member function 'std::__tree<std::pair<int, int>, std::less<std::pair<int, int>>, std::allocator<std::pair<int, int>>>::operator=' requested here
  666 |     __tree_ = __s.__tree_;
      |             ^
experimental/users/jgorbe/libcxx-repro/a.cc:8:6: note: in instantiation of member function 'std::set<std::pair<int, int>>::operator=' requested here
    8 |   s2 = s1;
      |      ^
1 error generated.
```

https://github.com/llvm/llvm-project/pull/134819