Let annotations on parameters see preceding type parameters

Fixes scala#12953

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

@@ -700,7 +700,7 @@ class Namer { typer: Typer =>
       case original: DefDef =>
         val typer1 = ctx.typer.newLikeThis
         nestedTyper(sym) = typer1
-        typer1.defDefSig(original, sym)(using localContext(sym).setTyper(typer1))
+        typer1.defDefSig(original, sym, this)(using localContext(sym).setTyper(typer1))
       case imp: Import =>
         try
           val expr1 = typedImportQualifier(imp, typedAheadExpr(_, _)(using ctx.withOwner(sym)))
@@ -733,6 +733,15 @@ class Namer { typer: Typer =>
             completer.complete(denot)
     }
 
+    private var completedTypeParamSyms: List[TypeSymbol] = null
+
+    def setCompletedTypeParams(tparams: List[TypeSymbol]) =
+      completedTypeParamSyms = tparams
+
+    override def completerTypeParams(sym: Symbol)(using Context): List[TypeSymbol] =
+      if completedTypeParamSyms != null then completedTypeParamSyms
+      else Nil
+
     protected def addAnnotations(sym: Symbol): Unit = original match {
       case original: untpd.MemberDef =>
         lazy val annotCtx = annotContext(original, sym)
@@ -1639,7 +1648,7 @@ class Namer { typer: Typer =>
   }
 
   /** The type signature of a DefDef with given symbol */
-  def defDefSig(ddef: DefDef, sym: Symbol)(using Context): Type = {
+  def defDefSig(ddef: DefDef, sym: Symbol, completer: Namer#Completer)(using Context): Type = {
     // Beware: ddef.name need not match sym.name if sym was freshened!
     val isConstructor = sym.name == nme.CONSTRUCTOR
 
@@ -1668,8 +1677,8 @@ class Namer { typer: Typer =>
     //   5. Info of CP is copied to DP and DP is completed.
     index(ddef.leadingTypeParams)
     if (isConstructor) sym.owner.typeParams.foreach(_.ensureCompleted())
-    for (tparam <- ddef.leadingTypeParams) typedAheadExpr(tparam)
-
+    completer.setCompletedTypeParams(
+      for tparam <- ddef.leadingTypeParams yield typedAheadExpr(tparam).symbol.asType)
     ddef.trailingParamss.foreach(completeParams)
     val paramSymss = normalizeIfConstructor(ddef.paramss.nestedMap(symbolOfTree), isConstructor)
     sym.setParamss(paramSymss)

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

@@ -2081,10 +2081,21 @@ class Typer extends Namer
   def annotContext(mdef: untpd.Tree, sym: Symbol)(using Context): Context = {
     def isInner(owner: Symbol) = owner == sym || sym.is(Param) && owner == sym.owner
     val outer = ctx.outersIterator.dropWhile(c => isInner(c.owner)).next()
-    outer.property(ExprOwner) match {
+    var adjusted = outer.property(ExprOwner) match {
       case Some(exprOwner) if outer.owner.isClass => outer.exprContext(mdef, exprOwner)
       case _ => outer
     }
+    sym.owner.infoOrCompleter match
+      case completer: Namer#Completer if sym.is(Param) =>
+        val tparams = completer.completerTypeParams(sym)
+        if tparams.nonEmpty then
+          // Create a new local context with a dummy owner and a scope containing the
+          // type parameters of the enclosing method or class. This annotations can see
+          // these type parameters. See i12953.scala for a test case.
+          val dummyOwner = newLocalDummy(sym.owner)
+          adjusted = adjusted.fresh.setOwner(dummyOwner).setScope(newScopeWith(tparams*))
+      case _ =>
+    adjusted
   }
 
   def completeAnnotations(mdef: untpd.MemberDef, sym: Symbol)(using Context): Unit = {

tests/pos/i12953.scala

@@ -0,0 +1,7 @@
+class Schema(impl: Class[_]) extends scala.annotation.StaticAnnotation
+
+class Ann[A] extends scala.annotation.StaticAnnotation
+
+case class Foo[A](@Schema(classOf[List[A]]) foo: String)
+case class Bar[A](@Ann[A] foo: String)
+def baz[A](@Ann[A] foo: String) = ()