Workaround #4987

Author: nicolasstucki

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

@@ -167,7 +167,11 @@ class ReifyQuotes extends MacroTransform {
       def pickleAsTasty() = {
         val meth =
           if (isType) ref(defn.Unpickler_unpickleType).appliedToType(originalTp)
-          else ref(defn.Unpickler_unpickleExpr).appliedToType(originalTp.widen.dealias)
+          else
+            val tpe =
+              if originalTp =:= defn.NilModule.termRef then originalTp // Workaround #4987
+              else originalTp.widen.dealias
+            ref(defn.Unpickler_unpickleExpr).appliedToType(tpe)
         val pickledQuoteStrings = liftList(PickledQuotes.pickleQuote(body).map(x => Literal(Constant(x))), defn.StringType)
         val splicesList = liftList(splices, defn.FunctionType(1).appliedTo(defn.SeqType.appliedTo(defn.AnyType), defn.AnyType))
         meth.appliedTo(pickledQuoteStrings, splicesList)

tests/pos-macros/nil-liftable.scala

@@ -0,0 +1,7 @@
+import scala.quoted._
+
+class Test:
+  given NilIsLiftable as Liftable[Nil.type] = new Liftable[Nil.type] {
+    def toExpr(xs: Nil.type): QuoteContext ?=> Expr[Nil.type] =
+      '{ Nil }
+  }