Fix #18816: Transfer the span of rewired This nodes in fullyParameterizedDef.

Author: sjrd

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

@@ -207,7 +207,7 @@ trait FullParameterization {
           .subst(origLeadingTypeParamSyms ++ origOtherParamSyms, (trefs ++ argRefs).tpes)
           .substThisUnlessStatic(origClass, thisRef.tpe),
         treeMap = {
-          case tree: This if tree.symbol == origClass => thisRef
+          case tree: This if tree.symbol == origClass => thisRef.withSpan(tree.span)
           case tree => rewireTree(tree, Nil) orElse tree
         },
         oldOwners = origMeth :: Nil,

tests/pos/i18816.scala

@@ -0,0 +1,12 @@
+trait Context
+
+class A(x: Context) extends AnyVal:
+  given [T]: Context = x
+
+  def m1 =
+    println(m3) // line 11
+    def m2 =
+      m3 // line 13
+    println(m2)
+
+  def m3(using Context): String = ""