Fix printing of OrderingConstraint

When printing a constraint, we print types which might
refer to type variables defined in the constraint, but the type printer
will rely on ctx.typerState.constraint to determine if these type
variables are instantiated, and this might be a different constraint
than the one we're trying to print, leading to an incorrect output.

This commit fixes this by temporarily setting ctx.typerState.constraint
to the current constraint when printing it, this required moving the
printing logic from OrderingConstraint to PlainPrinter.

At the same time, we drop the distinction between `toText` and
`contentsToString` (the former wrapped the printed output in
"Constraint(...)" and the latter didn't, now we never do) because
preserving it would have been complicated and it didn't seem worth it.

Also fix Ordering#toString to correctly print the bounds (the logic was
there but it was dead code).

Author: smarter

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

@@ -183,7 +183,4 @@ abstract class Constraint extends Showable {
    *  of athe type lambda that is associated with the typevar itself.
    */
   def checkConsistentVars()(using Context): Unit
-
-  /** A string describing the constraint's contents without a header or trailer */
-  def contentsToString(using Context): String
 }

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

@@ -641,49 +641,10 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     upperMap.foreachBinding((_, paramss) => paramss.foreach(_.foreach(checkClosedType(_, "upper"))))
   end checkClosed
 
-// ---------- toText -----------------------------------------------------
-
-  private def contentsToText(printer: Printer): Text =
-    //Printer.debugPrintUnique = true
-    def entryText(tp: Type) = tp match {
-      case tp: TypeBounds =>
-        tp.toText(printer)
-      case _ =>
-        " := " ~ tp.toText(printer)
-    }
-    val indent = 3
-    val uninstVarsText = " uninstantiated variables: " ~
-      Text(uninstVars.map(_.toText(printer)), ", ")
-    val constrainedText =
-      " constrained types: " ~ Text(domainLambdas map (_.toText(printer)), ", ")
-    val boundsText =
-      " bounds: " ~ {
-        val assocs =
-          for (param <- domainParams)
-          yield (" " * indent) ~ param.toText(printer) ~ entryText(entry(param))
-        Text(assocs, "\n")
-      }
-    val orderingText =
-      " ordering: " ~ {
-        val deps =
-          for {
-            param <- domainParams
-            ups = minUpper(param)
-            if ups.nonEmpty
-          }
-          yield
-            (" " * indent) ~ param.toText(printer) ~ " <: " ~
-              Text(ups.map(_.toText(printer)), ", ")
-        Text(deps, "\n")
-      }
-    //Printer.debugPrintUnique = false
-    Text.lines(List(uninstVarsText, constrainedText, boundsText, orderingText))
+// ---------- Printing -----------------------------------------------------
 
   override def toText(printer: Printer): Text =
-    Text.lines(List("Constraint(", contentsToText(printer), ")"))
-
-  def contentsToString(using Context): String =
-    contentsToText(ctx.printer).show
+    printer.toText(this)
 
   override def toString: String = {
     def entryText(tp: Type): String = tp match {
@@ -692,7 +653,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     }
     val constrainedText =
       " constrained types = " + domainLambdas.mkString("\n")
-    val boundsText = domainLambdas
+    val boundsText =
       " bounds = " + {
         val assocs =
           for (param <- domainParams)

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

@@ -599,6 +599,47 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case _ => "{...}"
     s"import $exprStr.$selectorStr"
 
+  def toText(c: OrderingConstraint): Text =
+    val savedConstraint = ctx.typerState.constraint
+    try
+      // The current TyperState constraint determines how type variables are printed
+      ctx.typerState.constraint = c
+      def entryText(tp: Type) = tp match {
+        case tp: TypeBounds =>
+          toText(tp)
+        case _ =>
+          " := " ~ toText(tp)
+      }
+      val indent = 3
+      val uninstVarsText = " uninstantiated variables: " ~
+        Text(c.uninstVars.map(toText), ", ")
+      val constrainedText =
+        " constrained types: " ~ Text(c.domainLambdas.map(toText), ", ")
+      val boundsText =
+        " bounds: " ~ {
+          val assocs =
+            for (param <- c.domainParams)
+            yield (" " * indent) ~ toText(param) ~ entryText(c.entry(param))
+          Text(assocs, "\n")
+        }
+      val orderingText =
+        " ordering: " ~ {
+          val deps =
+            for {
+              param <- c.domainParams
+              ups = c.minUpper(param)
+              if ups.nonEmpty
+            }
+            yield
+              (" " * indent) ~ toText(param) ~ " <: " ~
+                Text(ups.map(toText), ", ")
+          Text(deps, "\n")
+        }
+      //Printer.debugPrintUnique = false
+      Text.lines(List(uninstVarsText, constrainedText, boundsText, orderingText))
+    finally
+      ctx.typerState.constraint = savedConstraint
+
   def plain: PlainPrinter = this
 
   protected def keywordStr(text: String): String = coloredStr(text, SyntaxHighlighting.KeywordColor)

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

@@ -154,6 +154,9 @@ abstract class Printer {
   /** Textual representation of info relating to an import clause */
   def toText(result: ImportInfo): Text
 
+  /** Textual representation of a constraint */
+  def toText(c: OrderingConstraint): Text
+
   /** Render element within highest precedence */
   def toTextLocal(elem: Showable): Text =
     atPrec(DotPrec) { elem.toText(this) }

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

@@ -149,7 +149,7 @@ object ErrorReporting {
           "the empty constraint"
         else
           i"""a constraint with:
-             |${c.contentsToString}"""
+             |$c"""
       i"""
         |${TypeComparer.explained(_.isSubType(found, expected), header)}
         |