[committed] libstdc++: Add assertion for valid facet type arguments

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

LWG 436 confirmed that const-qualified types are valid arguments for
Facet template parameters, but volatile-qualified types are not.  Add an
assertion to locale::combine to check for valid types.

libstdc++-v3/ChangeLog:

        * include/bits/locale_classes.h (__is_facet): New helper.
        * include/bits/locale_classes.tcc (locale::combine): Check that
        _Facet type is valid.
---
 libstdc++-v3/include/bits/locale_classes.h   | 6 ++++++
 libstdc++-v3/include/bits/locale_classes.tcc | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/libstdc++-v3/include/bits/locale_classes.h 
b/libstdc++-v3/include/bits/locale_classes.h
index 50a748066f1..e168881b516 100644
--- a/libstdc++-v3/include/bits/locale_classes.h
+++ b/libstdc++-v3/include/bits/locale_classes.h
@@ -381,6 +381,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
   };
 
+#if __cpp_lib_type_trait_variable_templates // C++ >= 17
+  template<typename _Tp>
+    constexpr bool __is_facet = is_base_of_v<locale::facet, _Tp>;
+  template<typename _Tp>
+    constexpr bool __is_facet<volatile _Tp> = false;
+#endif
 
   // 22.1.1.1.2  Class locale::facet
   /**
diff --git a/libstdc++-v3/include/bits/locale_classes.tcc 
b/libstdc++-v3/include/bits/locale_classes.tcc
index d5ef1911057..1b5f5f2417c 100644
--- a/libstdc++-v3/include/bits/locale_classes.tcc
+++ b/libstdc++-v3/include/bits/locale_classes.tcc
@@ -71,6 +71,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     locale::
     combine(const locale& __other) const
     {
+#if __cpp_lib_type_trait_variable_templates // C++ >= 17
+      static_assert(__is_facet<_Facet>, "Template argument must be a facet");
+#endif
+
       _Impl* __tmp = new _Impl(*_M_impl, 1);
       __try
        {
-- 
2.46.0