libstdc++: Fix filesystem::path constraints for volatile [PR 100630]

The constraint check for filesystem::path construction uses
decltype(__is_path_src(declval<Source>())) which mean it considers
conversion from an rvalue.  When Source is a volatile-qualified type
it cannot use is_path_src(const Unknown&) because a const lvalue
reference can only bind to a non-volatile rvalue.

Since the relevant path members all have a const Source& parameter,
the constraint should be defined in terms of declval<const Source&>(),
not declval<Source>(). This avoids the problem of volatile-qualified
rvalues, because we no longer use an rvalue at all.

libstdc++-v3/ChangeLog:

	PR libstdc++/100630
	* include/experimental/bits/fs_path.h (__is_constructible_from):
	Test construction from a const lvalue, not an rvalue.
	* testsuite/27_io/filesystem/path/construct/100630.cc: New test.
	* testsuite/experimental/filesystem/path/construct/100630.cc:
	New test.

1 files changed, 14 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/construct/100630.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/construct/100630.cc
new file mode 100644
index 00000000000..eb8473e600e
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/construct/100630.cc
@@ -0,0 +1,14 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++17 } }
+
+#include <filesystem>
+
+void f(bool) { }
+void f(const std::filesystem::path&) { }
+
+void
+test_100630()
+{
+  volatile bool b = true;
+  f(b);
+}