CWG3190 Ambiguous lookup for type aliases in reflection

Author: burblebee

source/expressions.tex

@@ -6836,7 +6836,8 @@
 \end{example}
 \item
 Otherwise, if lookup finds a namespace alias\iref{namespace.alias},
-$R$ represents that namespace alias.
+all declarations found by name lookup shall have the same target scope, and
+$R$ represents the namespace alias.
 \item
 Otherwise, if lookup finds a namespace\iref{basic.namespace},
 $R$ represents that namespace.
@@ -6873,10 +6874,29 @@
 \end{note}
 \end{itemize}
 \item
-Otherwise, if lookup finds a type alias $A$,
-$R$ represents the underlying entity of $A$
-if $A$ was introduced by the declaration of a template parameter;
-otherwise, $R$ represents $A$.
+Otherwise, if lookup finds a type alias:
+\begin{itemize}
+\item
+If any declaration found by name lookup is of a template parameter $T$,
+$R$ represents the underlying entity of $T$.
+\item
+Otherwise,
+all declarations found by name lookup shall have the same target scope, and
+$R$ represents the type alias.
+\end{itemize}
+\begin{example}
+\begin{codeblock}
+namespace A {
+  using T = int;
+}
+namespace B {
+  using T = int;
+}
+using namespace A;
+using namespace B;
+auto r = ^^T;           // error: lookup finds type aliases with different target scopes
+\end{codeblock}
+\end{example}
 \item
 Otherwise, if lookup finds a class or an enumeration,
 $R$ represents the denoted type.