Workaround #4819: Avoid creating incorrect JointRefDenotations

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

@@ -490,7 +490,9 @@ object Denotations {
          *   4. The access boundary of sym2 is properly contained in the access
          *      boundary of sym1. For protected access, we count the enclosing
          *      package as access boundary.
-         *   5. sym1 a method but sym2 is not.
+         *   5. sym1 is a method but sym2 is not.
+         *   6. sym1 is a non-polymorphic method but sym2 is a polymorphic method.
+         *      (to be consistent with infoMeet, see #4819)
          *  The aim of these criteria is to give some disambiguation on access which
          *   - does not depend on textual order or other arbitrary choices
          *   - minimizes raising of doubleDef errors
@@ -505,6 +507,7 @@ object Denotations {
                 accessBoundary(sym2).isProperlyContainedIn(accessBoundary(sym1)) ||
                 sym2.is(Bridge) && !sym1.is(Bridge) ||
                 sym1.is(Method) && !sym2.is(Method)) ||
+                sym1.info.isInstanceOf[MethodType] && sym2.info.isInstanceOf[PolyType] ||
               sym1.info.isErroneous)
 
         /** Sym preference provided types also override */

tests/neg/i4819.scala

@@ -0,0 +1,16 @@
+trait One[X]  {
+  def concat(suffix: Int): X = ???
+}
+
+trait Two[Y <: Foo] {
+  def concat[Dummy](suffix: Int): Y = ???
+}
+
+class Foo extends One[Foo] with Two[Foo] {
+  concat(0) // OK
+
+  // TODO: This does not typecheck because the polymorphic overload is masked
+  // (we merge the denotations for both overloads into one and always prefer
+  // MethodType to PolyType, instead we should return a MultiDenotation). See #4819.
+  concat[Int](0) // error (that should actually not be an error)
+}

tests/pos/i4819.scala

@@ -0,0 +1,12 @@
+trait One[X]  {
+  def concat(suffix: Int): X = ???
+}
+
+trait Two[Y <: Foo] {
+  def concat[Dummy](suffix: Int): Y = ???
+}
+
+class Foo extends One[Foo] with Two[Foo] {
+  concat(0) // OK
+  // See also tests/neg/i4819.scala
+}