Add better explanation to error message

Fixes #18657

Author: odersky

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -876,7 +876,7 @@ extends Message(PatternMatchExhaustivityID) {
 
     val pathes = List(
       ActionPatch(
-        srcPos = endPos, 
+        srcPos = endPos,
         replacement = uncoveredCases.map(c => indent(s"case $c => ???", startColumn))
           .mkString("\n", "\n", "")
       ),
@@ -1806,10 +1806,20 @@ class NotAPath(tp: Type, usage: String)(using Context) extends TypeMsg(NotAPathI
         | - a reference to `this`, or
         | - a selection of an immutable path with an immutable value."""
 
-class WrongNumberOfParameters(expected: Int)(using Context)
+class WrongNumberOfParameters(tree: untpd.Tree, foundCount: Int, pt: Type, expectedCount: Int)(using Context)
   extends SyntaxMsg(WrongNumberOfParametersID) {
-  def msg(using Context) = s"Wrong number of parameters, expected: $expected"
-  def explain(using Context) = ""
+  def msg(using Context) = s"Wrong number of parameters, expected: $expectedCount"
+  def explain(using Context) =
+    val ending = if foundCount == 1 then "" else "s"
+    i"""The function literal
+       |
+       |    $tree
+       |
+       |has $foundCount parameter$ending. But the expected type
+       |
+       |    $pt
+       |
+       |requires a function with $expectedCount parameters."""
 }
 
 class DuplicatePrivateProtectedQualifier()(using Context)

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

@@ -1574,7 +1574,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     /** Returns the type and whether the parameter is erased */
     def protoFormal(i: Int): (Type, Boolean) =
       if (protoFormals.length == params.length) (protoFormals(i), isDefinedErased(i))
-      else (errorType(WrongNumberOfParameters(protoFormals.length), tree.srcPos), false)
+      else (errorType(WrongNumberOfParameters(tree, params.length, pt, protoFormals.length), tree.srcPos), false)
 
     /** Is `formal` a product type which is elementwise compatible with `params`? */
     def ptIsCorrectProduct(formal: Type) =

tests/neg/i18657.check

@@ -0,0 +1,17 @@
+-- [E086] Syntax Error: tests/neg/i18657.scala:2:27 --------------------------------------------------------------------
+2 |val f: (Int, Int) => Int = Integer.compare(_ + 1, _)  // error
+  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                           Wrong number of parameters, expected: 2
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | The function literal
+  |
+  |     _$2 => Integer.compare(_$1 => _$1 + 1, _$2)
+  |
+  | has 1 parameter. But the expected type
+  |
+  |     (Int, Int) => Int
+  |
+  | requires a function with 2 parameters.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/i18657.scala

@@ -0,0 +1,2 @@
+//> using options -explain
+val f: (Int, Int) => Int = Integer.compare(_ + 1, _)  // error