[libcxx-commits] [PATCH] D147741: [libc++, std::vector] call the optimized version of __uninitialized_allocator_copy for trivial types

[Ilya Biryukov via Phabricator via libcxx-commits]

ilya-biryukov added a comment.

We've noticed during our integrate that the following code (godbolt <https://gcc.godbolt.org/z/3doG8c784>) started producing a compiler error after this change:

  #include <vector>
  
  struct Value {};
  template <class Iter>
  struct WrappedIter : Iter {
      using pointer = Value*;
      using reference = Value&;
  
      pointer operator-> () { return &v; }
      reference operator* () { return v;  }
  
  private:
    Value v;
  };
  
  
  using WI = WrappedIter<std::vector<int>::iterator>;
  void foo(WI b, WI e) {
      std::vector<Value> v(b, e);
  }

Concretely, the call to vector constructor eventually gets to `std::__to_address` via `__unwrap_iter` and results in a compiler error:

  /opt/compiler-explorer/clang-trunk-20230602/bin/../include/c++/v1/__algorithm/unwrap_iter.h:46:32: error: no matching function for call to '__to_address'
  ...
  /opt/compiler-explorer/clang-trunk-20230602/bin/../include/c++/v1/__memory/pointer_traits.h:172:6: note: candidate template ignored: could not match '_Tp *' against 'WrappedIter<std::__wrap_iter<int *>>'
    172 | _Tp* __to_address(_Tp* __p) _NOEXCEPT {
        |      ^
  /opt/compiler-explorer/clang-trunk-20230602/bin/../include/c++/v1/__memory/pointer_traits.h:204:1: note: candidate template ignored: requirement 'integral_constant<bool, false>::value' was not satisfied [with _Pointer = WrappedIter<std::__wrap_iter<int *>>]
    204 | __to_address(const _Pointer& __p) _NOEXCEPT {
        | ^

The code itself is wrong as the wrapper (unintentionally) pretends to be a contiguous iterator, but fails to provide a proper `std::to_address` implementation (as the arrow function is not const).
Nevertheless, I wanted to sanity check that folks feel this new compiler error is standard-compliant. @hiraditya what are your thoughts on this?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D147741/new/

https://reviews.llvm.org/D147741