[committed] libstdc++: Add workaround for fs::path constraint recursion [PR106201]

[committed] libstdc++: Add workaround for fs::path constraint recursion [PR106201]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to gcc-12 branch.

-- >8 --

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.
---
 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 ++++++++++++++
 .../experimental/filesystem/iterators/106201.cc    | 14 ++++++++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/27_io/filesystem/iterators/106201.cc
 create mode 100644 libstdc++-v3/testsuite/experimental/filesystem/iterators/106201.cc

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> );
-- 
2.38.1