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

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: 290750

1 files changed, 15 insertions, 2 deletions

diff --git a/libcxx/include/ios b/libcxx/include/ios
index cbea478a74a..dfeb1f62eed 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -585,9 +585,22 @@ public:
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
 
+#if defined(_LIBCPP_CXX03_LANG)
+private:
+    struct __bool_tag {};
+    typedef void (basic_ios::*_BoolType)(__bool_tag) const;
+    void __true_value(__bool_tag) const {}
+
+public:
+    _LIBCPP_ALWAYS_INLINE
+    operator _BoolType() const {
+        return !fail() ? &basic_ios::__true_value : nullptr;
+    }
+#else
     _LIBCPP_ALWAYS_INLINE
-        _LIBCPP_EXPLICIT
-        operator bool() const {return !fail();}
+    _LIBCPP_EXPLICIT operator bool() const {return !fail();}
+#endif
+
     _LIBCPP_ALWAYS_INLINE bool operator!() const    {return  fail();}
     _LIBCPP_ALWAYS_INLINE iostate rdstate() const   {return ios_base::rdstate();}
     _LIBCPP_ALWAYS_INLINE void clear(iostate __state = goodbit) {ios_base::clear(__state);}