[gcc r9-9593] libstdc++: Fix filesystem::path constraints for volatile [PR 100630]

[Jonathan Wakely]

https://gcc.gnu.org/g:3c805829c9023da48db6d636221fd79612dff7ea

commit r9-9593-g3c805829c9023da48db6d636221fd79612dff7ea
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon May 17 11:54:06 2021 +0100

    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/bits/fs_path.h (__is_constructible_from): Test
            construction from a const lvalue, not an rvalue.
            * include/experimental/bits/fs_path.h (__is_constructible_from):
            Likewise.
            * testsuite/27_io/filesystem/path/construct/100630.cc: New test.
            * testsuite/experimental/filesystem/path/construct/100630.cc:
            New test.
    
    (cherry picked from commit 4cd69a5a0dd31bc6fdef1bbabc8d6d1416014ea1)

Diff:
---
 libstdc++-v3/include/bits/fs_path.h                        |  2 +-
 libstdc++-v3/include/experimental/bits/fs_path.h           |  2 +-
 .../testsuite/27_io/filesystem/path/construct/100630.cc    | 14 ++++++++++++++
 .../experimental/filesystem/path/construct/100630.cc       | 14 ++++++++++++++
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index 13eb18c0652..8b801c6787c 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -110,7 +110,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     template<typename _Source>
       struct __constructible_from<_Source, void>
-      : decltype(__is_path_src(std::declval<_Source>(), 0))
+      : decltype(__is_path_src(std::declval<const _Source&>(), 0))
       { };
 
     template<typename _Tp1, typename _Tp2 = void>
diff --git a/libstdc++-v3/include/experimental/bits/fs_path.h b/libstdc++-v3/include/experimental/bits/fs_path.h
index 33a415645d9..ba12b2f1734 100644
--- a/libstdc++-v3/include/experimental/bits/fs_path.h
+++ b/libstdc++-v3/include/experimental/bits/fs_path.h
@@ -125,7 +125,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     template<typename _Source>
       struct __constructible_from<_Source, void>
-      : decltype(__is_path_src(std::declval<_Source>(), 0))
+      : decltype(__is_path_src(std::declval<const _Source&>(), 0))
       { };
 
     template<typename _Tp1, typename _Tp2 = void,
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);
+}
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/path/construct/100630.cc b/libstdc++-v3/testsuite/experimental/filesystem/path/construct/100630.cc
new file mode 100644
index 00000000000..b2428ff74cf
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/path/construct/100630.cc
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+// { dg-require-filesystem-ts "" }
+
+#include <experimental/filesystem>
+
+void f(bool) { }
+void f(const std::experimental::filesystem::path&) { }
+
+void
+test_100630()
+{
+  volatile bool b = true;
+  f(b);
+}