Don't strip @uncheckedVariance when inferring the type of default getters

Fixes #12273

Author: odersky

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

@@ -164,7 +164,7 @@ object TypeOps:
         // corrective steps, so no widening is wanted.
         simplify(l, theMap) | simplify(r, theMap)
       case AnnotatedType(parent, annot)
-      if !ctx.mode.is(Mode.Type) && annot.symbol == defn.UncheckedVarianceAnnot =>
+      if annot.symbol == defn.UncheckedVarianceAnnot && !ctx.mode.is(Mode.Type) && !theMap.isInstanceOf[SimplifyKeepUnchecked] =>
         simplify(parent, theMap)
       case _: MatchType =>
         val normed = tp.tryNormalize
@@ -180,6 +180,8 @@ object TypeOps:
     def apply(tp: Type): Type = simplify(tp, this)
   }
 
+  class SimplifyKeepUnchecked(using Context) extends SimplifyMap
+
   /** Approximate union type by intersection of its dominators.
    *  That is, replace a union type Tn | ... | Tn
    *  by the smallest intersection type of base-class instances of T1,...,Tn.

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

@@ -1550,10 +1550,12 @@ class Namer { typer: Typer =>
           // For justification on the use of `@uncheckedVariance`, see
           // `default-getter-variance.scala`.
           AnnotatedType(defaultTp, Annotation(defn.UncheckedVarianceAnnot))
-        else tp.widenTermRefExpr.simplified match
-          case ctp: ConstantType if isInlineVal => ctp
-          case tp =>
-            TypeComparer.widenInferred(tp, pt)
+        else
+          // don't strip @uncheckedVariance annot for default getters
+          TypeOps.simplify(tp.widenTermRefExpr,
+              if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null) match
+            case ctp: ConstantType if isInlineVal => ctp
+            case tp => TypeComparer.widenInferred(tp, pt)
 
       // Replace aliases to Unit by Unit itself. If we leave the alias in
       // it would be erased to BoxedUnit.

tests/pos/i12273.scala

@@ -0,0 +1,4 @@
+import scala.annotation.unchecked.uncheckedVariance
+
+final case class Outlet[T](out: T)
+final case class SourceShape[+T](val out: Outlet[T @uncheckedVariance])