Fix #9517: Do not print recursively ErrorType on -Xprint-types

Author: nicolasstucki

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

@@ -4706,15 +4706,17 @@ object Types {
 
   object ErrorType:
     def apply(m: Message)(using Context): ErrorType =
-      val et = new ErrorType:
-        def msg(using Context): Message =
-          ctx.base.errorTypeMsg.get(this) match
-            case Some(m) => m
-            case None => "error message from previous run no longer available"
+      val et = new PreviousErrorType
       ctx.base.errorTypeMsg(et) = m
       et
   end ErrorType
 
+  class PreviousErrorType extends ErrorType:
+    def msg(using Context): Message =
+      ctx.base.errorTypeMsg.get(this) match
+        case Some(m) => m
+        case None => "error message from previous run no longer available"
+
   object UnspecifiedErrorType extends ErrorType {
     override def msg(using Context): Message = "unspecified error"
   }

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -177,6 +177,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
             keywordStr(" match ") ~ "{" ~ casesText ~ "}" ~
             (" <: " ~ toText(bound) provided !bound.isAny)
         }.close
+      case tp: PreviousErrorType =>
+        "<error>"
       case tp: ErrorType =>
         s"<error ${tp.msg.rawMessage}>"
       case tp: WildcardType =>

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -149,6 +149,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i1754.scala", allowDeepSubtypes),
+      compileFile("tests/neg-custom-args/i9517.scala", defaultOptions.and("-Xprint-types")),
       compileFile("tests/neg-custom-args/interop-polytypes.scala", allowDeepSubtypes.and("-Yexplicit-nulls")),
       compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")),
       compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings"),

tests/neg-custom-args/i9517.scala

@@ -0,0 +1,3 @@
+
+def test():Unit = foo({ case 1 => 10 }) // error
+def foo(x: Any): Boolean = true