[expr.const] Fix (variable) "usable in constant expressions" definition domain

Author: hubert-reinterpretcast

source/expressions.tex

@@ -8431,9 +8431,10 @@
 it has reference or non-volatile const-qualified integral or enumeration type.
 
 \pnum
-A constant-initialized potentially-constant variable $V$ is
+A variable $V$ is
 \defnx{usable in constant expressions}{usable in constant expressions!variable} at a point $P$ if
-$V$'s initializing declaration $D$ is reachable from $P$ and
+$V$ is constant-initialized and potentially-constant,
+$V$'s initializing declaration $D$ is reachable from $P$, and
 \begin{itemize}
 \item $V$ is constexpr,
 \item $V$ is not initialized to a TU-local value, or