libstdc++: Fix filesystem::path comparisons for C++23

In C++23 there is a basic_string_view(Range&&) constructor, which is
constrained to take a range (specifically, a contiguous_range). When the
filesystem::path comparison operators call lhs.compare(rhs) the overload
taking a string_view is considered, which means checking whether path
satisfies the range concept. That satisfaction result changes depending
whether path::iterator is complete, which is ill-formed; no diagnostic
required. To avoid the problem, this change ensures that the overload
resolution is performed in a context where path::iterator is complete
and the range concept is satisfied. (The result of overload resolution
is always that the compare(const path&) overload is the best match, but
we still have to consider the compare(basic_string_view<value_type>) one
to decide if it even participates in overload resolution).

For std::filesystem::path we can't define the comparison operators later
in the file, because they are hidden friends, so a new helper is
introduced that gets defined when everything else is complete.

For std::experimental::filesystem::path we can just move the definitions
of the comparison operators later in the file.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (operator==, operator<=>): Use new
	_S_compare function.
	(path::_S_compare): New function to call path::compare in a
	context where path::iterator is complete.
	* include/experimental/bits/fs_path.h (operator<, operator==):
	Define after path::iterator is complete.
	* testsuite/27_io/filesystem/path/native/conv_c++23.cc: New
	test.
	* testsuite/experimental/filesystem/path/native/conv_c++23.cc:
	New test.

libstdc++-v3/include/bits/fs_path.h

@@ -513,13 +513,13 @@ namespace __detail
 
     /// Compare paths
     friend bool operator==(const path& __lhs, const path& __rhs) noexcept
-    { return __lhs.compare(__rhs) == 0; }
+    { return path::_S_compare(__lhs, __rhs) == 0; }
 
 #if __cpp_lib_three_way_comparison
     /// Compare paths
     friend strong_ordering
     operator<=>(const path& __lhs, const path& __rhs) noexcept
-    { return __lhs.compare(__rhs) <=> 0; }
+    { return path::_S_compare(__lhs, __rhs) <=> 0; }
 #else
     /// Compare paths
     friend bool operator!=(const path& __lhs, const path& __rhs) noexcept
@@ -627,6 +627,11 @@ namespace __detail
       static basic_string<_CharT, _Traits, _Allocator>
       _S_str_convert(basic_string_view<value_type>, const _Allocator&);
 
+    // Returns lhs.compare(rhs), but defined after path::iterator is complete.
+    __attribute__((__always_inline__))
+    static int
+    _S_compare(const path& __lhs, const path& __rhs) noexcept;
+
     void _M_split_cmpts();
 
     _Type _M_type() const noexcept { return _M_cmpts.type(); }
@@ -1330,6 +1335,14 @@ namespace __detail
     return _M_at_end == __rhs._M_at_end;
   }
 
+  // Define this now that path and path::iterator are complete.
+  // It needs to consider the string_view(Range&&) constructor during
+  // overload resolution, which depends on whether range<path> is satisfied,
+  // which depends on whether path::iterator is complete.
+  inline int
+  path::_S_compare(const path& __lhs, const path& __rhs) noexcept
+  { return __lhs.compare(__rhs); }
+
   /// @} group filesystem
 _GLIBCXX_END_NAMESPACE_CXX11
 } // namespace filesystem

libstdc++-v3/include/experimental/bits/fs_path.h

@@ -551,8 +551,7 @@ namespace __detail
   size_t hash_value(const path& __p) noexcept;
 
   /// Compare paths
-  inline bool operator<(const path& __lhs, const path& __rhs) noexcept
-  { return __lhs.compare(__rhs) < 0; }
+  inline bool operator<(const path& __lhs, const path& __rhs) noexcept;
 
   /// Compare paths
   inline bool operator<=(const path& __lhs, const path& __rhs) noexcept
@@ -567,8 +566,7 @@ namespace __detail
   { return !(__lhs < __rhs); }
 
   /// Compare paths
-  inline bool operator==(const path& __lhs, const path& __rhs) noexcept
-  { return __lhs.compare(__rhs) == 0; }
+  inline bool operator==(const path& __lhs, const path& __rhs) noexcept;
 
   /// Compare paths
   inline bool operator!=(const path& __lhs, const path& __rhs) noexcept
@@ -1275,6 +1273,16 @@ namespace __detail
     return _M_at_end == __rhs._M_at_end;
   }
 
+  // Define these now that path and path::iterator are complete.
+  // They needs to consider the string_view(Range&&) constructor during
+  // overload resolution, which depends on whether range<path> is satisfied,
+  // which depends on whether path::iterator is complete.
+  inline bool operator<(const path& __lhs, const path& __rhs) noexcept
+  { return __lhs.compare(__rhs) < 0; }
+
+  inline bool operator==(const path& __lhs, const path& __rhs) noexcept
+  { return __lhs.compare(__rhs) == 0; }
+
   /// @} group filesystem-ts
 _GLIBCXX_END_NAMESPACE_CXX11
 } // namespace v1

libstdc++-v3/testsuite/27_io/filesystem/path/native/conv_c++23.cc

@@ -0,0 +1,12 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <filesystem>
+
+void
+test01()
+{
+  using std::filesystem::path;
+  path p;
+  path::string_type s(p);
+}

libstdc++-v3/testsuite/experimental/filesystem/path/native/conv_c++23.cc

@@ -0,0 +1,12 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <experimental/filesystem>
+
+void
+test01()
+{
+  using std::experimental::filesystem::path;
+  path p;
+  path::string_type s(p);
+}