Be more lazy when checking CaptureChecked annotation in TreeUnpickler

Fixes scala#15980

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

@@ -20,6 +20,8 @@ object Annotations {
 
     def symbol(using Context): Symbol = annotClass(tree)
 
+    def hasSymbol(sym: Symbol)(using Context) = symbol == sym
+
     def matches(cls: Symbol)(using Context): Boolean = symbol.derivesFrom(cls)
 
     def appliesToModule: Boolean = true // for now; see remark in SymDenotations
@@ -127,6 +129,11 @@ object Annotations {
     override def isEvaluated: Boolean = myTree.isInstanceOf[Tree @unchecked]
   }
 
+  class DeferredSymAndTree(symFn: Context ?=> Symbol, treeFn: Context ?=> Tree)
+  extends LazyAnnotation:
+    protected var mySym: Symbol | (Context ?=> Symbol) | Null = ctx ?=> symFn(using ctx)
+    protected var myTree: Tree | (Context ?=> Tree) | Null = ctx ?=> treeFn(using ctx)
+
   /** An annotation indicating the body of a right-hand side,
    *  typically of an inline method. Treated specially in
    *  pickling/unpickling and TypeTreeMaps
@@ -193,18 +200,15 @@ object Annotations {
       apply(New(atp, args))
 
     /** Create an annotation where the tree is computed lazily. */
-    def deferred(sym: Symbol)(treeFn: Context ?=> Tree)(using Context): Annotation =
+    def deferred(sym: Symbol)(treeFn: Context ?=> Tree): Annotation =
       new LazyAnnotation {
         protected var myTree: Tree | (Context ?=> Tree) | Null = ctx ?=> treeFn(using ctx)
         protected var mySym: Symbol | (Context ?=> Symbol) | Null = sym
       }
 
     /** Create an annotation where the symbol and the tree are computed lazily. */
-    def deferredSymAndTree(symFn: Context ?=> Symbol)(treeFn: Context ?=> Tree)(using Context): Annotation =
-      new LazyAnnotation {
-        protected var mySym: Symbol | (Context ?=> Symbol) | Null = ctx ?=> symFn(using ctx)
-        protected var myTree: Tree | (Context ?=> Tree) | Null = ctx ?=> treeFn(using ctx)
-      }
+    def deferredSymAndTree(symFn: Context ?=> Symbol)(treeFn: Context ?=> Tree): Annotation =
+      DeferredSymAndTree(symFn, treeFn)
 
     /** Extractor for child annotations */
     object Child {

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -624,7 +624,7 @@ class TreeUnpickler(reader: TastyReader,
             else
               newSymbol(ctx.owner, name, flags, completer, privateWithin, coord)
         }
-      val annots =  annotFns.map(_(sym.owner))
+      val annots = annotFns.map(_(sym.owner))
       sym.annotations = annots
       if sym.isOpaqueAlias then sym.setFlag(Deferred)
       val isScala2MacroDefinedInScala3 = flags.is(Macro, butNot = Inline) && flags.is(Erased)
@@ -642,7 +642,7 @@ class TreeUnpickler(reader: TastyReader,
       }
       registerSym(start, sym)
       if (isClass) {
-        if sym.owner.is(Package) && annots.exists(_.symbol == defn.CaptureCheckedAnnot) then
+        if sym.owner.is(Package) && annots.exists(_.hasSymbol(defn.CaptureCheckedAnnot)) then
           wasCaptureChecked = true
         sym.completer.withDecls(newScope)
         forkAt(templateStart).indexTemplateParams()(using localContext(sym))
@@ -737,7 +737,15 @@ class TreeUnpickler(reader: TastyReader,
       val tp = readType()
       val lazyAnnotTree = readLaterWithOwner(end, _.readTerm())
       owner =>
-        Annotation.deferredSymAndTree(tp.typeSymbol)(lazyAnnotTree(owner).complete)
+        new DeferredSymAndTree(tp.typeSymbol, lazyAnnotTree(owner).complete):
+          // Only force computation of symbol if it has the right name. This added
+          // amount of laziness is sometimes necessary to avid cycles. Test case pos/i15980.
+          override def hasSymbol(sym: Symbol)(using Context) = tp match
+            case tp: TypeRef =>
+              tp.designator match
+                case name: Name => name == sym.name && tp.symbol == sym
+                case _ => tp.symbol == sym
+            case _ => this.symbol == sym
 
     /** Create symbols for the definitions in the statement sequence between
      *  current address and `end`.

tests/new/test.scala

@@ -0,0 +1,2 @@
+object Test:
+  def test = 0

tests/pos/i15980/scalaql/1_syntax_1.scala

@@ -0,0 +1,6 @@
+package scalaql.syntax
+
+import scalaql.*
+
+@forbiddenInheritance
+trait ScalaqlSyntax

tests/pos/i15980/scalaql/2_annotations_1.scala

@@ -0,0 +1,5 @@
+package scalaql
+
+import scala.annotation.StaticAnnotation
+
+class forbiddenInheritance extends StaticAnnotation

tests/pos/i15980/scalaql/3_package_1.scala

@@ -0,0 +1,3 @@
+import scalaql.syntax.ScalaqlSyntax
+
+package object scalaql extends ScalaqlSyntax

tests/pos/i15980/scalaql/usage_2.scala

@@ -0,0 +1,4 @@
+package scalaql
+
+sealed trait Foo extends Product
+