Add "Printing" mode

Idea: when printing, we should be more lenient about conditions that
would otherwise cause an assertion failure, because we want to avoid
triggering further assertions while diagnosing previous errors. As a start
we generalize an assertion that RefinedTypes cannot be created after erasure.
This gets triggered when playing around with printing lambdas at erasure time
(see following commit): erasure runs at phase erasure + 1, so ctx.erasedTypes is true,
but we might still want to print lambdas then, and printing lambdas will create new
refined types as of next commit.

3 files changed, 5 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 3ab621db1..7d11ffae8 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -1842,7 +1842,7 @@ object Types {
       else make(RefinedType(parent, names.head, infoFns.head), names.tail, infoFns.tail)
 
     def apply(parent: Type, name: Name, infoFn: RefinedType => Type)(implicit ctx: Context): RefinedType = {
-      assert(!ctx.erasedTypes)
+      assert(!ctx.erasedTypes || ctx.mode.is(Mode.Printing))
       ctx.base.uniqueRefinedTypes.enterIfNew(new CachedRefinedType(parent, name, infoFn)).checkInst
     }
 
diff --git a/src/dotty/tools/dotc/printing/PlainPrinter.scala b/src/dotty/tools/dotc/printing/PlainPrinter.scala
index fb1c0fc74..9e4c43fec 100644
--- a/src/dotty/tools/dotc/printing/PlainPrinter.scala
+++ b/src/dotty/tools/dotc/printing/PlainPrinter.scala
@@ -8,10 +8,11 @@ import StdNames.nme
 import ast.Trees._, ast._
 import java.lang.Integer.toOctalString
 import config.Config.summarizeDepth
+import typer.Mode
 import scala.annotation.switch
 
 class PlainPrinter(_ctx: Context) extends Printer {
-  protected[this] implicit def ctx: Context = _ctx
+  protected[this] implicit def ctx: Context = _ctx.fresh.addMode(Mode.Printing)
 
   protected def maxToTextRecursions = 100
 
diff --git a/src/dotty/tools/dotc/typer/Mode.scala b/src/dotty/tools/dotc/typer/Mode.scala
index 997741819..b585cc793 100644
--- a/src/dotty/tools/dotc/typer/Mode.scala
+++ b/src/dotty/tools/dotc/typer/Mode.scala
@@ -63,5 +63,7 @@ object Mode {
    */
   val AllowDependentFunctions = newMode(9, "AllowDependentFunctions")
 
+  val Printing = newMode(10, "Printing")
+
   val PatternOrType = Pattern | Type
 }