Implement P0738

This addresses #35

stl/inc/iterator

@@ -133,17 +133,23 @@ public:
     using traits_type  = _Traits;
     using istream_type = basic_istream<_Elem, _Traits>;
 
+    static_assert(conjunction_v<is_default_constructible<_Ty>, is_copy_constructible<_Ty>, is_copy_assignable<_Ty>>,
+        "The type T shall meet the Cpp17DefaultConstructible, Cpp17CopyConstructible, and Cpp17CopyAssignable "
+        "requirements. (Enabled unconditionally from CXX20)");
+
     constexpr istream_iterator() : _Myistr(nullptr), _Myval() {}
 
-    istream_iterator(istream_type& _Istr) : _Myistr(_STD addressof(_Istr)) {
+    istream_iterator(istream_type& _Istr) : _Myistr(_STD addressof(_Istr)), _Myval() {
         _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);
     }
 
@@ -164,6 +170,7 @@ public:
 
 protected:
     void _Getval() { // get a _Ty value if possible
+        _STL_ASSERT(_Myistr, "The stored stream pointer in_stream must be non null");
         if (_Myistr && !(*_Myistr >> _Myval)) {
             _Myistr = nullptr;
         }

stl/inc/yvals_core.h

@@ -35,6 +35,7 @@
 // P0646R1 list/forward_list remove()/remove_if()/unique() Return size_type
 // P0653R2 to_address()
 // P0655R1 visit<R>()
+// P0738R2 istream_iterator Cleanup. The minstrel know it as "I Stream, You Stream, We All Stream for istream_iterator"
 // P0758R1 is_nothrow_convertible
 // P0768R1 Library Support For The Spaceship Comparison Operator <=>
 //     (partially implemented)
@@ -158,6 +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