Merge pull request #11553 from dotty-staging/fix-11544

Improve error message in overloading resolution

Author: odersky

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

@@ -1779,7 +1779,6 @@ class Typer extends Namer
         tpt1.tpe.typeSymbol == defn.PolyFunctionClass && rsym.name == nme.apply
       if (!polymorphicRefinementAllowed && rsym.info.isInstanceOf[PolyType] && rsym.allOverriddenSymbols.isEmpty)
         report.error(PolymorphicMethodMissingTypeInParent(rsym, tpt1.symbol), refinement.srcPos)
-
       val member = refineCls.info.member(rsym.name)
       if (member.isOverloaded)
         report.error(OverloadInRefinement(rsym), refinement.srcPos)
@@ -3124,20 +3123,16 @@ class Typer extends Namer
               }
           }
         case ambiAlts =>
-          if (tree.tpe.isErroneous || pt.isErroneous) tree.withType(UnspecifiedErrorType)
-          else {
+          if tree.tpe.isErroneous || pt.isErroneous then tree.withType(UnspecifiedErrorType)
+          else
             val remainingDenots = altDenots.filter(denot => ambiAlts.contains(altRef(denot)))
             val addendum =
-              if ambiAlts.toSet.size != ambiAlts.size then
-                // Several variants have the same signature. This can happen for structural
-                // type selections. See i8736.scala
-                """|
-                   |
-                   |Note: this happens because two or more alternatives have the same erasure,
-                   |      so they cannot be distinguished by overloading resolution""".stripMargin
+              if ambiAlts.exists(!_.symbol.exists) then
+                i"""|
+                    |
+                    |Note: Overloaded definitions introduced by refinements cannot be resolved"""
               else ""
             errorTree(tree, AmbiguousOverload(tree, remainingDenots, pt, addendum))
-          }
       }
     }
 

tests/neg/i11544.check

@@ -0,0 +1,11 @@
+-- [E051] Reference Error: tests/neg/i11544.scala:6:13 -----------------------------------------------------------------
+6 |  val n = m1.foo(23)  // error
+  |          ^^^^^^
+  |          Ambiguous overload. The overloaded alternatives of method (str: String, int: Int): Int with types
+  |           (str: String, int: Int): Int
+  |           (arg: Int): Int
+  |          both match arguments ((23 : Int))
+  |
+  |          Note: Overloaded definitions introduced by refinements cannot be resolved
+
+longer explanation available when compiling with `-explain`

tests/neg/i11544.scala

@@ -0,0 +1,7 @@
+import language.reflectiveCalls
+
+object Foo {
+  val m: ({ def foo(str: String, int: Int): Int }){ def foo(arg: Int): Int } = ???
+  val m1 = m
+  val n = m1.foo(23)  // error
+}

tests/neg/i8736.check

@@ -17,7 +17,6 @@
    |                 (k: ("z" : String)): Boolean
    |                all match arguments (("z" : String))
    |
-   |                Note: this happens because two or more alternatives have the same erasure,
-   |                      so they cannot be distinguished by overloading resolution
+   |                Note: Overloaded definitions introduced by refinements cannot be resolved
 
 longer explanation available when compiling with `-explain`