Improve error messages for overloading errors

We need to show argument lists after the first one if they were
used for overloading resolution.

Author: odersky

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

@@ -69,15 +69,28 @@ object ErrorReporting {
         "\n(Note that variables need to be initialized to be defined)"
       else ""
 
+    /** Reveal arguments in FunProtos that are proteted by an IgnoredProto but were
+     *  revealed during type inference. This gives clearer error messages for overloading
+     *  resolution errors that need to show argument lists after the first. We do not
+     *  reveal other kinds of ignored prototypes since these might be misleading because
+     *  there might be a possible implicit conversion on the result.
+     */
+    def revealDeepenedArgs(tp: Type): Type = tp match
+      case tp @ IgnoredProto(deepTp: FunProto) if tp.wasDeepened => deepTp
+      case _ => tp
+
     def expectedTypeStr(tp: Type): String = tp match {
       case tp: PolyProto =>
-        em"type arguments [${tp.targs.tpes}%, %] and ${expectedTypeStr(tp.resultType)}"
+        em"type arguments [${tp.targs.tpes}%, %] and ${expectedTypeStr(revealDeepenedArgs(tp.resultType))}"
       case tp: FunProto =>
-        val result = tp.resultType match {
-          case _: WildcardType | _: IgnoredProto => ""
-          case tp => em" and expected result type $tp"
-        }
-        em"arguments (${tp.typedArgs().tpes}%, %)$result"
+        def argStr(tp: FunProto): String =
+          val result = revealDeepenedArgs(tp.resultType) match {
+            case restp: FunProto => argStr(restp)
+            case _: WildcardType | _: IgnoredProto => ""
+            case tp => em" and expected result type $tp"
+          }
+          em"(${tp.typedArgs().tpes}%, %)$result"
+        s"arguments ${argStr(tp)}"
       case _ =>
         em"expected type $tp"
     }
@@ -125,25 +138,25 @@ object ErrorReporting {
     def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailureType = NoMatchingImplicits): Tree = {
       val normTp = normalize(tree.tpe, pt)
       val normPt = normalize(pt, pt)
-      
+
       def contextFunctionCount(tp: Type): Int = tp.stripped match
         case defn.ContextFunctionType(_, restp, _) => 1 + contextFunctionCount(restp)
         case _ => 0
       def strippedTpCount = contextFunctionCount(tree.tpe) - contextFunctionCount(normTp)
       def strippedPtCount = contextFunctionCount(pt) - contextFunctionCount(normPt)
-      
+
       val (treeTp, expectedTp) =
         if normTp <:< normPt || strippedTpCount != strippedPtCount
         then (tree.tpe, pt)
         else (normTp, normPt)
         // use normalized types if that also shows an error, and both sides stripped
         // the same number of context functions. Use original types otherwise.
-        
+
       def missingElse = tree match
         case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
           "\nMaybe you are missing an else part for the conditional?"
         case _ => ""
-        
+
       errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), implicitFailure.whyNoConversion, missingElse))
     }
 

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

@@ -131,10 +131,18 @@ object ProtoTypes {
 
   /** A class marking ignored prototypes that can be revealed by `deepenProto` */
   abstract case class IgnoredProto(ignored: Type) extends CachedGroundType with MatchAlways:
+    private var myWasDeepened = false
     override def revealIgnored = ignored
-    override def deepenProto(using Context): Type = ignored
+    override def deepenProto(using Context): Type =
+      myWasDeepened = true
+      ignored
     override def deepenProtoTrans(using Context): Type = ignored.deepenProtoTrans
 
+    /** Did someone look inside via deepenProto? Used for error deagniostics
+     *  to give a more extensive expected type.
+     */
+    def wasDeepened: Boolean = myWasDeepened
+
     override def computeHash(bs: Hashable.Binders): Int = doHash(bs, ignored)
 
     override def eql(that: Type): Boolean = that match

tests/neg/i10901.check

@@ -15,7 +15,7 @@
    |                 (x: T1)
    |                   (y: BugExp4Point2D.ColumnType[T2])
    |                     (implicit evidence$5: Numeric[T1], evidence$6: Numeric[T2]): BugExp4Point2D.Point2D[T1, T2]
-   |               both match arguments ((x : BugExp4Point2D.IntT.type))
+   |               both match arguments ((x : BugExp4Point2D.IntT.type))((y : BugExp4Point2D.DoubleT.type))
 -- [E008] Not Found Error: tests/neg/i10901.scala:48:38 ----------------------------------------------------------------
 48 |    val pos4: Point2D[Int,Double] = x º 201.1   // error
    |                                    ^^^
@@ -29,7 +29,7 @@
    |          (x: BugExp4Point2D.ColumnType[T1])
    |            (y: T2)(implicit evidence$9: Numeric[T1], evidence$10: Numeric[T2]): BugExp4Point2D.Point2D[T1, T2]
    |         [T1, T2](x: T1)(y: T2)(implicit evidence$3: Numeric[T1], evidence$4: Numeric[T2]): BugExp4Point2D.Point2D[T1, T2]
-   |        both match arguments ((x : BugExp4Point2D.IntT.type))
+   |        both match arguments ((x : BugExp4Point2D.IntT.type))((201.1d : Double))
 -- [E008] Not Found Error: tests/neg/i10901.scala:62:16 ----------------------------------------------------------------
 62 |  val y = "abc".foo  // error
    |          ^^^^^^^^^

tests/neg/i15000.check

@@ -21,4 +21,4 @@
    |        Ambiguous overload. The overloaded alternatives of method apply in object ExtensionMethodReproduction with types
    |         (c: ExtensionMethodReproduction.C)(x: Int, y: Int): String
    |         (c: ExtensionMethodReproduction.C)(x: Int, y: String): String
-   |        both match arguments (ExtensionMethodReproduction.c.type)
+   |        both match arguments (ExtensionMethodReproduction.c.type)((ExtensionMethodReproduction.x : Int), <error Not found: barY>)

tests/neg/i15287.check

@@ -0,0 +1,7 @@
+-- [E134] Type Error: tests/neg/i15287.scala:4:19 ----------------------------------------------------------------------
+4 |@main def Test() = f('c')(2) // error
+  |                   ^
+  |                   None of the overloaded alternatives of method f with types
+  |                    (x: Char)(min: Boolean, max: Int): String
+  |                    (x: Char)(someParam: String): String
+  |                   match arguments (('c' : Char))((2 : Int))

tests/neg/i15287.scala

@@ -0,0 +1,4 @@
+def f(x: Char)(someParam: String): String = "a"
+def f(x: Char)(min: Boolean, max: Int = 4): String = "b"
+
+@main def Test() = f('c')(2) // error

tests/run/i15287.scala

@@ -1,4 +1,9 @@
-def f(x: Char)(someParam: String): String = "a"
-def f(x: Char)(min: Boolean, max: Int = 4): String = "b"
+extension (x: Char)
+  def f(someParam: String): String = "a"
+  def f(min: Boolean, max: Int = 4): String = "b"
+
+@main def Test() =
+  f('c')(false)
+  'c'.f(true)
+  'c'.f("a")
 
-@main def Test() = f('c')(false)