Improve our ability to override default parameters

Don't just use the type of the rhs of the default getter as its inferred
type, this can be more precise than the type of the corresponding
parameter and prevent seemingly valid overrides.

Author: smarter

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

@@ -1439,7 +1439,11 @@ class Namer { typer: Typer =>
         val defaultTp = defaultParamType
         val pt = inherited.orElse(wildApprox(defaultTp)).orElse(WildcardType)
         val tp = typedAheadRhs(pt.widenExpr).tpe
-        tp.widenTermRefExpr.simplified match
+        if (defaultTp eq pt) && (tp frozen_<:< defaultTp) then
+          // When possible, widen to the default getter parameter type to permit a
+          // larger choice of overrides (see `default-getter.scala`).
+          defaultTp
+        else tp.widenTermRefExpr.simplified match
           case ctp: ConstantType if isInlineVal => ctp
           case tp =>
             TypeComparer.widenInferred(tp, pt)

tests/pos/default-getter.scala

@@ -0,0 +1,10 @@
+class X
+class Y extends X
+
+class A {
+  def foo(param: X = new Y): X = param
+}
+
+class B extends A {
+  override def foo(param: X = new X): X = param
+}