Fix #8058: Fix isStableMember test

isStableMember classified NoSymbol as stable. This meant that
an Application without a symbol was classified as pure in TreeInfo.
This in turn meant that the application could be dropped and replaced
with `Unit` at the end of a block. The (non-)pure application in
the test was an array update.

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -727,7 +727,7 @@ object SymDenotations {
       */
     final def isStableMember(implicit ctx: Context): Boolean = {
       def isUnstableValue = isOneOf(UnstableValueFlags) || info.isInstanceOf[ExprType]
-      isType || is(StableRealizable) || !isUnstableValue
+      isType || is(StableRealizable) || exists && !isUnstableValue
     }
 
     /** Is this a denotation of a class that does not have - either direct or inherited -

tests/run/i8058.scala

@@ -0,0 +1,10 @@
+extension on (x: Array[Char]):
+  inline def swap(i: Int, j: Int) : Unit =
+    val v = x(i)
+    x(i)  = x(j)
+    x(j)  = v
+
+@main def Test =
+  val a = Array('A','B')
+  a.swap(0, 1)
+  assert(a.toList == List('B', 'A'))