[class.mi] Define ambiguous/unambiguous base class

Currently, terms _ambiguous base class_ and _unambiguous base class_
are used without clear definition. The intent seems already clear to me,
but perhaps it would be better to properly define them.

Author: frederick-vs-ja

source/classes.tex

@@ -3665,6 +3665,27 @@
 \end{importgraphic}
 \end{note}
 
+\pnum
+\indextext{base class!ambiguous}%
+\indextext{base class!unambiguous}%
+When class \tcode{B} is a possibly indirect base class of class \tcode{D},
+if there is only one possibly indirect base class subobject of type \tcode{B}
+of a \tcode{D} object, \tcode{B} is an \defnadj{unambiguous}{base class} of \tcode{D},
+otherwise, \tcode{B} is an \defnadj{ambiguous}{base class} of \tcode{D}.
+\begin{example}
+\begin{codeblock}
+class B { @\commentellip@ };
+class MX : public B { @\commentellip@ };
+class MY : public B { @\commentellip@ };
+class D1 : public MX, public MY { @\commentellip@ };      // \tcode{B} is an ambiguous base class of \tcode{D1}
+class D2 : public MX, public B { @\commentellip@ };       // \tcode{B} is an ambiguous base class of \tcode{D2}
+
+class VMX : virtual public B { @\commentellip@ };
+class VMY : virtual public B { @\commentellip@ };
+class D3 : public VMX, public VMY { @\commentellip@ };    // \tcode{B} is an unambiguous base class of \tcode{D3}
+\end{codeblock}
+\end{example}
+
 \rSec2[class.virtual]{Virtual functions}%
 \indextext{function!virtual|(}%
 \indextext{type!polymorphic}%