[gcc r12-8927] libstdc++: Add workaround for fs::path constraint recursion [PR106201]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:477b3f9d58589b3af7a5a7d038d78352ad66406e

commit r12-8927-g477b3f9d58589b3af7a5a7d038d78352ad66406e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 22 18:15:56 2022 +0000

    libstdc++: Add workaround for fs::path constraint recursion [PR106201]
    
    This works around a compiler bug where overload resolution attempts
    implicit conversion to path in order to call a function with a path&
    parameter. Such conversion would produce a prvalue, which would not be
    able to bind to the lvalue reference anyway. Attempting to check the
    conversion causes a constraint recursion because the arguments to the
    path constructor are checked to see if they're iterators, which checks
    if they're swappable, which tries to use the swap function that
    triggered the conversion in the first place.
    
    This replaces the swap function with an abbreviated function template
    that is constrained with same_as<path> auto& so that the invalid
    conversion is never considered.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/106201
            * include/bits/fs_path.h (filesystem::swap(path&, path&)):
            Replace with abbreviated function template.
            * include/experimental/bits/fs_path.h (filesystem::swap):
            Likewise.
            * testsuite/27_io/filesystem/iterators/106201.cc: New test.
            * testsuite/experimental/filesystem/iterators/106201.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/fs_path.h                        |  7 +++++++
 libstdc++-v3/include/experimental/bits/fs_path.h           |  9 ++++++++-
 .../testsuite/27_io/filesystem/iterators/106201.cc         | 14 ++++++++++++++
 .../testsuite/experimental/filesystem/iterators/106201.cc  | 14 ++++++++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index 65682c2a185..1b4a1b69f37 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -737,7 +737,14 @@ namespace __detail
   /// @{
   /// @relates std::filesystem::path
 
+#if __cpp_concepts >= 201907L
+  // Workaround for PR libstdc++/106201
+  inline void
+  swap(same_as<path> auto& __lhs, same_as<path> auto& __rhs) noexcept
+  { __lhs.swap(__rhs); }
+#else
   inline void swap(path& __lhs, path& __rhs) noexcept { __lhs.swap(__rhs); }
+#endif
 
   size_t hash_value(const path& __p) noexcept;
 
diff --git a/libstdc++-v3/include/experimental/bits/fs_path.h b/libstdc++-v3/include/experimental/bits/fs_path.h
index a493e17a37e..ba6acb2158d 100644
--- a/libstdc++-v3/include/experimental/bits/fs_path.h
+++ b/libstdc++-v3/include/experimental/bits/fs_path.h
@@ -537,7 +537,14 @@ namespace __detail
   /// @relates std::experimental::filesystem::path @{
 
   /// Swap overload for paths
-  inline void swap(path& __lhs, path& __rhs) noexcept { __lhs.swap(__rhs); }
+#if __cpp_concepts >= 201907L
+  // Workaround for PR libstdc++/106201
+  inline void
+  swap(same_as<path> auto& __lhs, same_as<path> auto& __rhs) noexcept
+  { __lhs.swap(__rhs); }
+#else
+   inline void swap(path& __lhs, path& __rhs) noexcept { __lhs.swap(__rhs); }
+#endif
 
   /// Compute a hash value for a path
   size_t hash_value(const path& __p) noexcept;
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/iterators/106201.cc b/libstdc++-v3/testsuite/27_io/filesystem/iterators/106201.cc
new file mode 100644
index 00000000000..c5fefd9ac3f
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/iterators/106201.cc
@@ -0,0 +1,14 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-filesystem-ts "" }
+
+// PR libstdc++/106201 constraint recursion in path(Source const&) constructor.
+
+#include <filesystem>
+#include <iterator>
+#include <concepts>
+namespace fs = std::filesystem;
+using I = std::counted_iterator<fs::directory_iterator>;
+static_assert( std::swappable<I> );
+using R = std::counted_iterator<fs::recursive_directory_iterator>;
+static_assert( std::swappable<R> );
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/iterators/106201.cc b/libstdc++-v3/testsuite/experimental/filesystem/iterators/106201.cc
new file mode 100644
index 00000000000..017b72ef5f6
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/iterators/106201.cc
@@ -0,0 +1,14 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-filesystem-ts "" }
+
+// PR libstdc++/106201 constraint recursion in path(Source const&) constructor.
+
+#include <experimental/filesystem>
+#include <iterator>
+#include <concepts>
+namespace fs = std::experimental::filesystem;
+using I = std::counted_iterator<fs::directory_iterator>;
+static_assert( std::swappable<I> );
+using R = std::counted_iterator<fs::recursive_directory_iterator>;
+static_assert( std::swappable<R> );