Improve missing argument list error

Fixes scala#17123

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

@@ -190,6 +190,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case InlineGivenShouldNotBeFunctionID // errorNumber 174
   case ValueDiscardingID // errorNumber 175
   case UnusedNonUnitValueID // errorNumber 176
+  case MissingArgumentListID // errorNumber: 177
 
   def errorNumber = ordinal - 1
 

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

@@ -1821,6 +1821,22 @@ class MissingEmptyArgumentList(method: String)(using Context)
   }
 }
 
+class MissingArgumentList(method: String, sym: Symbol)(using Context)
+  extends SyntaxMsg(MissingArgumentListID) {
+  def msg(using Context) =
+    def paramsStr(tp: Type): String = tp match
+      case tp: MethodType if !tp.isImplicitMethod =>
+        tp.paramNames.zip(tp.paramInfos)
+          .map((name, tpe) => name.show + ": " + tpe.show)
+          .mkString("(", ", ", ")" + paramsStr(tp.resType))
+      case tp: MethodOrPoly => paramsStr(tp.resType)
+      case _ => ""
+    i"missing argument list for $method\n${sym} must be called with arguments ${hl(paramsStr(sym.info))}"
+  def explain(using Context) = {
+    i"""Unapplied methods are only converted to functions when a function type is expected."""
+  }
+}
+
 class DuplicateNamedTypeParameter(name: Name)(using Context)
   extends SyntaxMsg(DuplicateNamedTypeParameterID) {
   def msg(using Context) = i"Type parameter $name was defined multiple times."

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

@@ -41,12 +41,22 @@ object ErrorReporting {
     errorType(WrongNumberOfTypeArgs(fntpe, expectedArgs, actual), pos)
 
   def missingArgs(tree: Tree, mt: Type)(using Context): Unit =
+    def isCallableWithoutArgumentsLists(mt: Type): Boolean = mt match
+        case pt: PolyType => isCallableWithoutArgumentsLists(pt.resType)
+        case mt: MethodType if mt.isImplicitMethod => isCallableWithoutArgumentsLists(mt.resType)
+        case mt: MethodType => false
+        case _ => true
+    def isCallableWithSingleEmptyArgumentList(mt: Type): Boolean =
+      mt match
+        case mt: MethodType if mt.paramNames.isEmpty => isCallableWithoutArgumentsLists(mt.resType)
+        case mt: MethodType if mt.isImplicitMethod => isCallableWithSingleEmptyArgumentList(mt.resType)
+        case pt: PolyType => isCallableWithSingleEmptyArgumentList(pt.resType)
+        case _ => false
     val meth = err.exprStr(methPart(tree))
-    mt match
-      case mt: MethodType if mt.paramNames.isEmpty =>
-        report.error(MissingEmptyArgumentList(meth), tree.srcPos)
-      case _ =>
-        report.error(em"missing arguments for $meth", tree.srcPos)
+    if isCallableWithSingleEmptyArgumentList(tree.symbol.info) then
+      report.error(MissingEmptyArgumentList(meth), tree.srcPos)
+    else
+      report.error(MissingArgumentList(meth, tree.symbol), tree.srcPos)
 
   def matchReductionAddendum(tps: Type*)(using Context): String =
     val collectMatchTrace = new TypeAccumulator[String]:

tests/neg/i17123.check

@@ -0,0 +1,76 @@
+-- [E100] Syntax Error: tests/neg/i17123.scala:7:2 ---------------------------------------------------------------------
+7 |  m1 // error
+  |  ^^
+  |  method m1 in object ConfusingErrorMessage must be called with () argument
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:9:2 ---------------------------------------------------------------------
+9 |  m2 // error
+  |  ^^
+  |  missing argument list for method m2 in object ConfusingErrorMessage
+  |  method m2 must be called with arguments ()()
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:10:4 --------------------------------------------------------------------
+10 |  m2() // error
+   |  ^^^^
+   |  missing argument list for method m2 in object ConfusingErrorMessage
+   |  method m2 must be called with arguments ()()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:11:2 --------------------------------------------------------------------
+11 |  m3 // error
+   |  ^^
+   |  missing argument list for method m3 in object ConfusingErrorMessage
+   |  method m3 must be called with arguments ()()()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:12:4 --------------------------------------------------------------------
+12 |  m3() // error
+   |  ^^^^
+   |  missing argument list for method m3 in object ConfusingErrorMessage
+   |  method m3 must be called with arguments ()()()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:13:6 --------------------------------------------------------------------
+13 |  m3()() // error
+   |  ^^^^^^
+   |  missing argument list for method m3 in object ConfusingErrorMessage
+   |  method m3 must be called with arguments ()()()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:15:2 --------------------------------------------------------------------
+15 |  f3 // error
+   |  ^^
+   |  missing argument list for method f3 in object ConfusingErrorMessage
+   |  method f3 must be called with arguments ()(i: Int)()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:16:2 --------------------------------------------------------------------
+16 |  f3() // error
+   |  ^^^^
+   |  missing argument list for method f3 in object ConfusingErrorMessage
+   |  method f3 must be called with arguments ()(i: Int)()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:17:6 --------------------------------------------------------------------
+17 |  f3()(2) // error
+   |  ^^^^^^^
+   |  missing argument list for method f3 in object ConfusingErrorMessage
+   |  method f3 must be called with arguments ()(i: Int)()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:19:2 --------------------------------------------------------------------
+19 |  i3 // error
+   |  ^^
+   |  missing argument list for method i3 in object ConfusingErrorMessage
+   |  method i3 must be called with arguments ()()
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E177] Syntax Error: tests/neg/i17123.scala:20:2 --------------------------------------------------------------------
+20 |  i3() // error
+   |  ^^^^
+   |  missing argument list for method i3 in object ConfusingErrorMessage
+   |  method i3 must be called with arguments ()()
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i17123.scala

@@ -0,0 +1,22 @@
+object ConfusingErrorMessage {
+  def m1() = ()
+  def m2()() = ()
+  def m3()()() = ()
+  def f3()(i: Int)() = ()
+  def i3()(using d: DummyImplicit)() = ()
+  m1 // error
+  m1()
+  m2 // error
+  m2() // error
+  m3 // error
+  m3() // error
+  m3()() // error
+  m3()()()
+  f3 // error
+  f3() // error
+  f3()(2) // error
+  f3()(2)()
+  i3 // error
+  i3() // error
+  i3()()
+}