Fix isFullyDefined trace (#16748)

First, switch to the "i" interpolator and replace "IFT" with
"isFullyDefined".  Then apply it to the whole apply, and make the
dealias appear in the trace.  Requires fixing an early return usage.
Also expose the ForceDegree specified.

Found this in the course of enabling tracing and the typr printer and
seeing lots of verbose type case classes printed.

Author: odersky

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

@@ -90,6 +90,7 @@ object Formatting {
       given Show[util.SourceFile]                     = ShowAny
       given Show[util.Spans.Span]                     = ShowAny
       given Show[tasty.TreeUnpickler#OwnerTree]       = ShowAny
+      given Show[typer.ForceDegree.Value]             = ShowAny
 
       private def show1[A: Show](x: A)(using Context) = show2(Show[A].show(x).ctxShow)
       private def show2(x: Shown)(using Context): String = x match

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

@@ -166,14 +166,18 @@ object Inferencing {
 
     private var toMaximize: List[TypeVar] = Nil
 
-    def apply(x: Boolean, tp: Type): Boolean =
-      try tp.dealias match
+    def apply(x: Boolean, tp: Type): Boolean = trace(i"isFullyDefined($tp, $force)", typr) {
+      try {
+      val tpd = tp.dealias
+      if tpd ne tp then apply(x, tpd)
+      else tp match
         case _: WildcardType | _: ProtoType =>
           false
         case tvar: TypeVar if !tvar.isInstantiated =>
           force.appliesTo(tvar)
           && ctx.typerState.constraint.contains(tvar)
           && {
+            var fail = false
             val direction = instDirection(tvar.origin)
             if minimizeSelected then
               if direction <= 0 && tvar.hasLowerBound then
@@ -186,17 +190,16 @@ object Inferencing {
             else if variance >= 0 && (force.ifBottom == IfBottom.ok && !tvar.hasUpperBound || tvar.hasLowerBound) then
               instantiate(tvar, fromBelow = true)
             else if variance >= 0 && force.ifBottom == IfBottom.fail then
-              return false
+              fail = true
             else
               toMaximize = tvar :: toMaximize
-            foldOver(x, tvar)
-          }
-        case tp =>
-          reporting.trace(s"IFT $tp") {
-            foldOver(x, tp)
+            !fail && foldOver(x, tvar)
           }
+        case tp => foldOver(x, tp)
+      }
       catch case ex: Throwable =>
         handleRecursive("check fully defined", tp.show, ex)
+    }
 
     def process(tp: Type): Boolean =
       // Maximize type vars in the order they were visited before */
@@ -767,13 +770,14 @@ trait Inferencing { this: Typer =>
   end constrainIfDependentParamRef
 }
 
-/** An enumeration controlling the degree of forcing in "is-dully-defined" checks. */
+/** An enumeration controlling the degree of forcing in "is-fully-defined" checks. */
 @sharable object ForceDegree {
-  class Value(val appliesTo: TypeVar => Boolean, val ifBottom: IfBottom)
-  val none: Value = new Value(_ => false, IfBottom.ok)
-  val all: Value = new Value(_ => true, IfBottom.ok)
-  val failBottom: Value = new Value(_ => true, IfBottom.fail)
-  val flipBottom: Value = new Value(_ => true, IfBottom.flip)
+  class Value(val appliesTo: TypeVar => Boolean, val ifBottom: IfBottom):
+    override def toString = s"ForceDegree.Value(.., $ifBottom)"
+  val none: Value       = new Value(_ => false, IfBottom.ok)  { override def toString = "ForceDegree.none" }
+  val all: Value        = new Value(_ => true, IfBottom.ok)   { override def toString = "ForceDegree.all" }
+  val failBottom: Value = new Value(_ => true, IfBottom.fail) { override def toString = "ForceDegree.failBottom" }
+  val flipBottom: Value = new Value(_ => true, IfBottom.flip) { override def toString = "ForceDegree.flipBottom" }
 }
 
 enum IfBottom: