P0738R2 istream_iterator Cleanup (#246)

This change is unconditional.

Resolves #35.

stl/inc/iterator

@@ -133,17 +133,23 @@ public:
     using traits_type  = _Traits;
     using istream_type = basic_istream<_Elem, _Traits>;
 
-    constexpr istream_iterator() : _Myistr(nullptr), _Myval() {}
+    static_assert(conjunction_v<is_default_constructible<_Ty>, is_copy_constructible<_Ty>, is_copy_assignable<_Ty>>,
+        "istream_iterator<T> requires T to be default constructible, copy constructible, and copy assignable. "
+        "(N4835 [istream.iterator]/2)");
+
+    constexpr istream_iterator() {}
 
     istream_iterator(istream_type& _Istr) : _Myistr(_STD addressof(_Istr)) {
         _Getval();
     }
 
     _NODISCARD const _Ty& operator*() const {
+        _STL_ASSERT(_Myistr, "The stored stream pointer in_stream must be non-null");
         return _Myval;
     }
 
     _NODISCARD const _Ty* operator->() const {
+        _STL_ASSERT(_Myistr, "The stored stream pointer in_stream must be non-null");
         return _STD addressof(_Myval);
     }
 
@@ -162,15 +168,16 @@ public:
         return _Myistr == _Right._Myistr;
     }
 
-protected:
+private:
     void _Getval() { // get a _Ty value if possible
-        if (_Myistr && !(*_Myistr >> _Myval)) {
+        _STL_ASSERT(_Myistr, "The stored stream pointer in_stream must be non-null");
+        if (!(*_Myistr >> _Myval)) {
             _Myistr = nullptr;
         }
     }
 
-    istream_type* _Myistr; // pointer to input stream
-    _Ty _Myval; // lookahead value (valid if _Myistr is not null)
+    istream_type* _Myistr{nullptr}; // pointer to input stream
+    _Ty _Myval{}; // lookahead value (valid if _Myistr is not null)
 };
 
 template <class _Ty, class _Elem, class _Traits, class _Diff>

stl/inc/yvals_core.h

@@ -159,6 +159,7 @@
 // P0548R1 Tweaking common_type And duration
 // P0558R1 Resolving atomic<T> Named Base Class Inconsistencies
 // P0599R1 noexcept hash
+// P0738R2 istream_iterator Cleanup
 // P0771R1 noexcept For std::function's Move Constructor
 // P0777R1 Avoiding Unnecessary decay
 // P0809R0 Comparing Unordered Containers