Recommit r290750: Fix PR19460 - std::ios is convertible to int.

There were two problems with the initial fix.

1. The added tests flushed out that we misconfigured _LIBCPP_EXPLICIT with GCC.

2. Because the boolean type was a member function template it caused weird link
   errors. I'm assuming due to the vague linkage rules. This time the bool type
   is a non-template member function pointer. That seems to have fixed the
   failing tests. Plus it will end up generating less symbols overall, since
   the bool type is no longer per instantiation.

original commit message below
-----------------------------

std::basic_ios has an operator bool(). In C++11 and later
it is explicit, and only allows contextual implicit conversions.

However explicit isn't available in C++03 which causes std::istream (et al)
to have an implicit conversion to int. This can easily cause ambiguities
when calling operator<< and operator>>.

This patch uses a "bool-like" type in C++03 to work around this. The
"bool-like" type is an arbitrary pointer to member function type. It
will not convert to either int or void*, but will convert to bool.

llvm-svn: 290754

1 files changed, 9 insertions, 0 deletions

diff --git a/libcxx/test/std/input.output/iostreams.base/ios/iostate.flags/bool.pass.cpp b/libcxx/test/std/input.output/iostreams.base/ios/iostate.flags/bool.pass.cpp
index 0de889e549c..4e3faa27c40 100644
--- a/libcxx/test/std/input.output/iostreams.base/ios/iostate.flags/bool.pass.cpp
+++ b/libcxx/test/std/input.output/iostreams.base/ios/iostate.flags/bool.pass.cpp
@@ -14,12 +14,21 @@
 // operator unspecified-bool-type() const;
 
 #include <ios>
+#include <type_traits>
 #include <cassert>
 
+#include "test_macros.h"
+
 int main()
 {
     std::ios ios(0);
     assert(static_cast<bool>(ios) == !ios.fail());
     ios.setstate(std::ios::failbit);
     assert(static_cast<bool>(ios) == !ios.fail());
+    static_assert((!std::is_convertible<std::ios, void*>::value), "");
+    static_assert((!std::is_convertible<std::ios, int>::value), "");
+    static_assert((!std::is_convertible<std::ios const&, int>::value), "");
+#if TEST_STD_VER >= 11
+    static_assert((!std::is_convertible<std::ios, bool>::value), "");
+#endif
 }