Fix #8391: Track locally defined @Alpha aliases only for definitions

Author: anatoliykmetyuk

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -1129,7 +1129,7 @@ trait Checking {
         val preExisting = ctx.effectiveScope.lookup(ename)
         if (preExisting.exists || seen.contains(ename))
           ctx.error(em"@alpha annotation ${'"'}$ename${'"'} clashes with other definition is same scope", stat.sourcePos)
-        seen += ename
+        if stat.isDef then seen += ename
       }
     }
   }

tests/pos/i8391.scala

@@ -0,0 +1,13 @@
+import scala.annotation.alpha
+
+trait Foo {
+  @alpha("intersection")
+  def *(other: Foo): Foo
+}
+
+@main def Test() = {
+  val s: Foo = ???
+  s * s
+  s * s
+  s * s
+}