[libcxx-commits] [libcxx] [libc++] Implement LWG3430 disallow implicit conversion of the source arguments to `std::filesystem::path` when constructing `std::basic_*fstream` (PR #85079)

[Nico Weber via libcxx-commits]

nico wrote:

This breaks existing code, e.g. this:

```
  std::fstream model_file(ml_package_dir()
                              .Append(kMlPackageDataDir)
                              .Append(kMlPackageModelFileName)
                              .value(),
                          std::ios::out | std::ios::binary);
```

`.value()` returns a `std::wstring`. (This is a Windows-only thing, which is probably why @ldionne didn't encounter it.)

Is the assumption that this will be rarely hit in practice?

<details>
<summary>full diag text</summary>

```
stderr:
../../services/webnn/coreml/graph_builder.cc(586,16): error: no matching constructor for initialization of 'std::fstream' (aka 'basic_fstream<char>')
  586 |   std::fstream model_file(ml_package_dir()
      |                ^          ~~~~~~~~~~~~~~~~
  587 |                               .Append(kMlPackageDataDir)
      |                               ~~~~~~~~~~~~~~~~~~~~~~~~~~
  588 |                               .Append(kMlPackageModelFileName)
      |                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  589 |                               .value(),
      |                               ~~~~~~~~~
  590 |                           std::ios::out | std::ios::binary);
      |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../third_party/libc++/src/include\fstream(1411,34): note: candidate constructor not viable: no known conversion from 'const StringType' (aka 'const basic_string<wchar_t>') to 'const char *' for 1st argument
 1411 |   _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(const char* __s,
      |                                  ^             ~~~~~~~~~~~~~~~
../../third_party/libc++/src/include\fstream(1414,34): note: candidate constructor not viable: no known conversion from 'const StringType' (aka 'const basic_string<wchar_t>') to 'const wchar_t *' for 1st argument
 1414 |   _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(const wchar_t* __s,
      |                                  ^             ~~~~~~~~~~~~~~~~~~
../../third_party/libc++/src/include\fstream(1417,34): note: candidate constructor not viable: no known conversion from 'const basic_string<wchar_t>' to 'const basic_string<char>' for 1st argument
 1417 |   _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(const string& __s,
      |                                  ^             ~~~~~~~~~~~~~~~~~
../../third_party/libc++/src/include\fstream(1422,74): note: candidate template ignored: requirement 'is_same_v<std::wstring, std::filesystem::path>' was not satisfied [with _Tp = StringType]
 1422 |   _LIBCPP_AVAILABILITY_FILESYSTEM_LIBRARY _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(
      |                                                                          ^
../../third_party/libc++/src/include\fstream(1427,25): note: candidate constructor not viable: requires single argument '__rhs', but 2 arguments were provided
 1427 |   _LIBCPP_HIDE_FROM_ABI basic_fstream(basic_fstream&& __rhs);
      |                         ^             ~~~~~~~~~~~~~~~~~~~~~
../../third_party/libc++/src/include\__fwd/fstream.h(28,28): note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
   28 | class _LIBCPP_TEMPLATE_VIS basic_fstream;
      |                            ^~~~~~~~~~~~~
../../third_party/libc++/src/include\fstream(1410,25): note: candidate constructor not viable: requires 0 arguments, but 2 were provided
 1410 |   _LIBCPP_HIDE_FROM_ABI basic_fstream();
      |                         ^
```
</details>

(https://source.chromium.org/chromium/chromium/src/+/main:services/webnn/coreml/graph_builder.cc;l=586?q=services%2Fwebnn%2Fcoreml%2Fgraph_builder.cc)

https://github.com/llvm/llvm-project/pull/85079