Keep HasDefault flag updated in overriding methods

Fixes #16006

#16006 started failing since we now count the number of default parameters
for better precision in overloading resolution. But that count was wrong since
it did not take inherited default getters from overridden methods into account.
We fix this by setting the HasDefault flag also for parameters that don't have
a default value themselves, but that correspond to parameters with default
values in overridden methods.

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

@@ -938,15 +938,27 @@ object SymDenotations {
 
     /** Does this symbol have defined or inherited default parameters?
      *  Default parameters are recognized until erasure.
+     *  As a side effect, sets HasDefault flag of parameters in methods that
+     *  override a method with a corresponding default parameter.
      */
     def hasDefaultParams(using Context): Boolean =
       if ctx.erasedTypes then false
       else if is(HasDefaultParams) then true
       else if is(NoDefaultParams) || !is(Method) then false
       else
-        val result =
-          rawParamss.nestedExists(_.is(HasDefault))
-          || allOverriddenSymbols.exists(_.hasDefaultParams)
+        val inheritedDefaultParamNames = util.HashSet[Name]()
+        for prev <- allOverriddenSymbols do
+          if prev.hasDefaultParams then
+            for params <- prev.rawParamss do
+              for param <- params do
+                if param.is(HasDefault) then inheritedDefaultParamNames += param.name
+        if !inheritedDefaultParamNames.isEmpty then
+          for params <- rawParamss do
+            for param <- params do
+              if inheritedDefaultParamNames.contains(param.name) then
+                param.setFlag(HasDefault)
+        val result = rawParamss.nestedExists(_.is(HasDefault))
+        if allOverriddenSymbols.exists(_.hasDefaultParams) && !ctx.isAfterTyper then assert(result)
         setFlag(if result then HasDefaultParams else NoDefaultParams)
         result
 

tests/run/i16006.check

@@ -0,0 +1 @@
+overriden (3, 10)

tests/run/i16006.scala

@@ -0,0 +1,11 @@
+class X:
+  def toString(maxLines: Int = 10, maxWidth: Int = 10): String = (maxLines -> maxWidth).toString
+
+class Foo extends X:
+  override def toString(maxLines: Int, maxWidth: Int): String = s"overriden ($maxLines, $maxWidth)"
+  override def toString(): String = toString(maxLines = 3)
+
+
+@main def Test = {
+  println(Foo().toString())
+}