[PATCH] libstdc++: Make std::any_cast ill-formed (LWG 3305)

[Jonathan Wakely]

Tested x86_64-linux.

-- >8 --

LWG 3305 was approved earlier this year in Tokyo. We need to give an
error if using std::any_cast<void>, but std::any_cast<void()> is valid
(but always returns null).

libstdc++-v3/ChangeLog:

        * include/std/any (any_cast(any*), any_cast(const any*)): Add
        static assertion to reject void types, as per LWG 3305.
        * testsuite/20_util/any/misc/lwg3305.cc: New test.
---
 libstdc++-v3/include/std/any                      |  8 ++++++++
 .../testsuite/20_util/any/misc/lwg3305.cc         | 15 +++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/20_util/any/misc/lwg3305.cc

diff --git a/libstdc++-v3/include/std/any b/libstdc++-v3/include/std/any
index 690ddc2aa57..e4709b1ce04 100644
--- a/libstdc++-v3/include/std/any
+++ b/libstdc++-v3/include/std/any
@@ -554,6 +554,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _ValueType>
     inline const _ValueType* any_cast(const any* __any) noexcept
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3305. any_cast<void>
+      static_assert(!is_void_v<_ValueType>);
+
+      // As an optimization, don't bother instantiating __any_caster for
+      // function types, since std::any can only hold objects.
       if constexpr (is_object_v<_ValueType>)
        if (__any)
          return static_cast<_ValueType*>(__any_caster<_ValueType>(__any));
@@ -563,6 +569,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _ValueType>
     inline _ValueType* any_cast(any* __any) noexcept
     {
+      static_assert(!is_void_v<_ValueType>);
+
       if constexpr (is_object_v<_ValueType>)
        if (__any)
          return static_cast<_ValueType*>(__any_caster<_ValueType>(__any));
diff --git a/libstdc++-v3/testsuite/20_util/any/misc/lwg3305.cc 
b/libstdc++-v3/testsuite/20_util/any/misc/lwg3305.cc
new file mode 100644
index 00000000000..49f5d747ab3
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/any/misc/lwg3305.cc
@@ -0,0 +1,15 @@
+// { dg-do compile { target c++17 } }
+
+// LWG 3305. any_cast<void>
+
+#include <any>
+
+void
+test_lwg3305()
+{
+  std::any a;
+  (void) std::any_cast<const void>(&a); // { dg-error "here" }
+  const std::any a2;
+  (void) std::any_cast<volatile void>(&a2); // { dg-error "here" }
+}
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
-- 
2.45.2