Fix #8058: Fix isStableMember test

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -442,7 +442,9 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   /** The purity level of this reference.
    *  @return
-   *    PurePath        if reference is (nonlazy and stable) or to a parameterized function
+   *    PurePath        if reference is (nonlazy and stable)
+   *                    or to a parameterized function
+   *                    or its type is a constant type
    *    IdempotentPath  if reference is lazy and stable
    *    Impure          otherwise
    *  @DarkDimius: need to make sure that lazy accessor methods have Lazy and Stable
@@ -452,6 +454,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     val sym = tree.symbol
     if (!tree.hasType) Impure
     else if (!tree.tpe.widen.isParameterless || sym.isEffectivelyErased) PurePath
+    else if tree.tpe.isInstanceOf[ConstantType] then PurePath
     else if (!sym.isStableMember) Impure
     else if (sym.is(Module))
       if (sym.moduleClass.isNoInitsClass) PurePath else IdempotentPath

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 -

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

@@ -1082,7 +1082,7 @@ object Types {
     /** Widen type if it is unstable (i.e. an ExprType, or TermRef to unstable symbol */
     final def widenIfUnstable(implicit ctx: Context): Type = stripTypeVar match {
       case tp: ExprType => tp.resultType.widenIfUnstable
-      case tp: TermRef if !tp.symbol.isStableMember => tp.underlying.widenIfUnstable
+      case tp: TermRef if tp.symbol.exists && !tp.symbol.isStableMember => tp.underlying.widenIfUnstable
       case _ => this
     }
 

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'))