<xstring>: Comment why basic_string::_Is_elem_cptr can't be a variable template (microsoft#2980)

Co-authored-by: Igor Zhukov <[email protected]>

Author: YairBorn

stl/inc/xstring

@@ -2479,6 +2479,8 @@ private:
     static constexpr size_t _Memcpy_val_size   = sizeof(_Scary_val) - _Memcpy_val_offset;
 
     template <class _Iter>
+    // TRANSITION, /clr:pure is incompatible with templated static constexpr data members
+    // static constexpr bool _Is_elem_cptr =_Is_any_of_v<_Iter, const _Elem* const, _Elem* const, const _Elem*, _Elem*>;
     using _Is_elem_cptr = bool_constant<_Is_any_of_v<_Iter, const _Elem* const, _Elem* const, const _Elem*, _Elem*>>;
 
 #if _HAS_CXX17