Add accessible check for import usage (#23348)

Fixes #23347 

The code for deciding whether a selector is used to import a symbol was
missing a check for accessibility (importability).

3.6.4 doesn't warn, so it would be nice to know why it excludes the
member. The algorithm is different but the code for choosing import
selectors is similar. 3.7.0 also doesn't warn, but no change stands out
as a culprit. Possibly the selector was spuriously excluded (by result
caching in 3.7.0).

Author: tgodzik

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -305,8 +305,10 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     def matchingSelector(info: ImportInfo): ImportSelector | Null =
       val qtpe = info.site
       def hasAltMember(nm: Name) = qtpe.member(nm).hasAltWith: alt =>
-           alt.symbol == sym
-        || nm.isTypeName && alt.symbol.isAliasType && alt.info.dealias.typeSymbol == sym
+        val sameSym =
+             alt.symbol == sym
+          || nm.isTypeName && alt.symbol.isAliasType && alt.info.dealias.typeSymbol == sym
+        sameSym && alt.symbol.isAccessibleFrom(qtpe)
       def loop(sels: List[ImportSelector]): ImportSelector | Null = sels match
         case sel :: sels =>
           val matches =

tests/warn/i23347.scala

@@ -0,0 +1,22 @@
+//> using options -Wunused:all
+
+object USED {
+  case class A(value: Int)
+}
+
+object UNUSED {
+  // In reality UNUSED would contain several other necessary members!
+  private type A = USED.A // warn private
+  class B
+}
+
+object Test {
+  import USED.*
+  import UNUSED.*
+
+  def foo(a: A): Int = a.value
+
+  def g(b: B) = ()
+
+}
+