Handle pickled forward references in pickled expressions

To compute forward references it was assumed that the owner of that
symbol can be found in the TASTy. This is not always the case with TASTy
expressions of pickled quotes. The owner might be outside the quote, in
this case the context already has the owner of the referenced symbol.
These are local symbols defined at the top-level of the TASTy.

Fixes #16843

Author: nicolasstucki

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

@@ -1620,8 +1620,9 @@ class TreeUnpickler(reader: TastyReader,
         try cs match {
           case ot :: cs1 =>
             if (ot.addr.index == addr.index) {
-              assert(current.exists, i"no symbol at $addr")
-              current
+              // If current does not exist it is beacause this is a pickled expression
+              // and the owner is located outside the expression (i.e. pickled quote).
+              current.orElse(ctx.owner)
             }
             else if (ot.addr.index < addr.index && addr.index < ot.end.index)
               search(ot.children, reader.symbolAt(ot.addr))

tests/pos-macros/i16843/Macro_1.scala

@@ -0,0 +1,18 @@
+import scala.quoted.*
+
+inline def foo: Int = ${ fooImpl }
+
+def fooImpl(using Quotes): Expr[Int] =
+  '{
+    val b = ${
+      val a = '{
+        (1: Int) match
+          case x @ (y: Int) => 0
+      }
+      a
+    }
+
+    (1: Int) match
+      case x @ (y: Int) => 0
+  }
+

tests/pos-macros/i16843/Test_2.scala

@@ -0,0 +1 @@
+def test = foo