CWG3081 Require glvalue when splicing direct base class relationship

Fixes NB US 29-062 (C++26 CD).

Author: eisenwave

source/basic.tex

@@ -4810,7 +4810,9 @@
 \item
   a class member access\iref{expr.ref} using the \tcode{.} operator
   where the left operand is one of these expressions and
-  the right operand designates a non-static data member of non-reference type,
+  the right operand designates
+  a non-static data member\iref{class.mem.general} of non-reference type or
+  a direct base class relationship\iref{class.derived.general},
 \item
   a pointer-to-member operation\iref{expr.mptr.oper} using the \tcode{.*} operator
   where the left operand is one of these expressions and

source/expressions.tex

@@ -4173,7 +4173,9 @@
 For a dot that is followed by an expression
 that designates a static member or an enumerator,
 the first expression is a discarded-value expression\iref{expr.context};
-if the expression after the dot designates a non-static data member,
+if the expression after the dot designates a
+non-static data member\iref{class.mem.general} or
+a direct base class relationship\iref{class.derived.general},
 the first expression shall be a glvalue.
 A postfix expression that is followed by an arrow
 shall be a prvalue having pointer type.