diff --git a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
index d1e91799f31a..bd9195f77ab3 100644
--- a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -170,14 +170,15 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
     case _ => false
   }
 
-  /** Is this argument node of the form <expr> : _* ?
+  /** Is this argument node of the form <expr> : _*, or is it a reference to
+   *  such an argument ? The latter case can happen when an argument is lifted.
    */
   def isWildcardStarArg(tree: Tree)(implicit ctx: Context): Boolean = unbind(tree) match {
     case Typed(Ident(nme.WILDCARD_STAR), _) => true
     case Typed(_, Ident(tpnme.WILDCARD_STAR)) => true
-    case Typed(_, tpt: TypeTree) => tpt.hasType && tpt.tpe.isRepeatedParam
+    case Typed(_, tpt: TypeTree) => tpt.typeOpt.isRepeatedParam
     case NamedArg(_, arg) => isWildcardStarArg(arg)
-    case _ => false
+    case arg => arg.typeOpt.widen.isRepeatedParam
   }
 
   /** If this tree has type parameters, those.  Otherwise Nil.
@@ -317,7 +318,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *            Idempotent  if running the statement a second time has no side effects
    *            Impure      otherwise
    */
-  private def statPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
+  def statPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
     case EmptyTree
        | TypeDef(_, _)
        | Import(_, _)
@@ -342,7 +343,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *  takes a different code path than all to follow; but they are idempotent
    *  because running the expression a second time gives the cached result.
    */
-  private def exprPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
+  def exprPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
     case EmptyTree
        | This(_)
        | Super(_, _)
@@ -397,7 +398,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *  @DarkDimius: need to make sure that lazy accessor methods have Lazy and Stable
    *               flags set.
    */
-  private def refPurity(tree: Tree)(implicit ctx: Context): PurityLevel =
+  def refPurity(tree: Tree)(implicit ctx: Context): PurityLevel =
     if (!tree.tpe.widen.isParameterless || tree.symbol.is(Erased)) SimplyPure
     else if (!tree.symbol.isStable) Impure
     else if (tree.symbol.is(Lazy)) Idempotent // TODO add Module flag, sinxce Module vals or not Lazy from the start.
diff --git a/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala b/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
index 0e6d98f2e3e4..6500693da068 100644
--- a/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
+++ b/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
@@ -218,6 +218,8 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         return "FunProto(" ~ argsText ~ "):" ~ toText(resultType)
       case tp: IgnoredProto =>
         return "?"
+      case tp @ PolyProto(targs, resType) =>
+        return "PolyProto(" ~ toTextGlobal(targs, ", ") ~ "): " ~ toText(resType)
       case _ =>
     }
     super.toText(tp)
diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index e6886b99e431..8f700481f111 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -80,7 +80,8 @@ class TreeChecker extends Phase with SymTransformer {
 
     if (sym.isClass && !sym.isAbsent) {
       val validSuperclass = sym.isPrimitiveValueClass || defn.syntheticCoreClasses.contains(sym) ||
-        (sym eq defn.ObjectClass) || (sym is NoSuperClass) || (sym.asClass.superClass.exists)
+        (sym eq defn.ObjectClass) || (sym is NoSuperClass) || (sym.asClass.superClass.exists) ||
+        sym.isRefinementClass
 
       assert(validSuperclass, i"$sym has no superclass set")
       testDuplicate(sym, seenClasses, "class")
@@ -97,7 +98,10 @@ class TreeChecker extends Phase with SymTransformer {
   def phaseName: String = "Ycheck"
 
   def run(implicit ctx: Context): Unit = {
-    check(ctx.allPhases, ctx)
+    if (ctx.settings.YtestPickler.value && ctx.phase.prev.isInstanceOf[Pickler])
+      ctx.echo("Skipping Ycheck after pickling with -Ytest-pickler, the returned tree contains stale symbols")
+    else
+      check(ctx.allPhases, ctx)
   }
 
   private def previousPhases(phases: List[Phase])(implicit ctx: Context): List[Phase] = phases match {
@@ -305,7 +309,7 @@ class TreeChecker extends Phase with SymTransformer {
 
     override def typedIdent(tree: untpd.Ident, pt: Type)(implicit ctx: Context): Tree = {
       assert(tree.isTerm || !ctx.isAfterTyper, tree.show + " at " + ctx.phase)
-      assert(tree.isType || !needsSelect(tree.tpe), i"bad type ${tree.tpe} for $tree # ${tree.uniqueId}")
+      assert(tree.isType || ctx.mode.is(Mode.Pattern) && untpd.isWildcardArg(tree) || !needsSelect(tree.tpe), i"bad type ${tree.tpe} for $tree # ${tree.uniqueId}")
       assertDefined(tree)
 
       checkNotRepeated(super.typedIdent(tree, pt))
@@ -443,7 +447,8 @@ class TreeChecker extends Phase with SymTransformer {
       if (ctx.mode.isExpr &&
           !tree.isEmpty &&
           !isPrimaryConstructorReturn &&
-          !pt.isInstanceOf[FunProto])
+          !pt.isInstanceOf[FunProto] &&
+          !pt.isInstanceOf[PolyProto])
         assert(tree.tpe <:< pt, {
           val mismatch = err.typeMismatchMsg(tree.tpe, pt)
           i"""|${mismatch.msg}
diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index 698fdaff0c15..742d5c41942e 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -13,6 +13,7 @@ import StdNames._
 import NameOps._
 import Symbols._
 import Trees._
+import TreeInfo._
 import ProtoTypes._
 import Constants._
 import Scopes._
@@ -601,17 +602,20 @@ trait Checking {
     }
   }
 
-  /** Check that `tree` is a pure expression of constant type */
-  def checkInlineConformant(tree: Tree, what: => String)(implicit ctx: Context): Unit =
+  /** Check that `tree` can be marked `inline` */
+  def checkInlineConformant(tree: Tree, isFinal: Boolean, what: => String)(implicit ctx: Context): Unit = {
+    // final vals can be marked inline even if they're not pure, see Typer#patchFinalVals
+    val purityLevel = if (isFinal) Idempotent else Pure
     tree.tpe match {
       case tp: TermRef if tp.symbol.is(InlineParam) => // ok
       case tp => tp.widenTermRefExpr match {
-        case tp: ConstantType if isPureExpr(tree) => // ok
-        case tp if defn.isFunctionType(tp) && isPureExpr(tree) => // ok
+        case tp: ConstantType if exprPurity(tree) >= purityLevel => // ok
+        case tp if defn.isFunctionType(tp) && exprPurity(tree) >= purityLevel => // ok
         case _ =>
           if (!ctx.erasedTypes) ctx.error(em"$what must be a constant expression or a function", tree.pos)
       }
     }
+  }
 
   /** Check that class does not declare same symbol twice */
   def checkNoDoubleDeclaration(cls: Symbol)(implicit ctx: Context): Unit = {
@@ -867,7 +871,7 @@ trait NoChecking extends ReChecking {
   override def checkClassType(tp: Type, pos: Position, traitReq: Boolean, stablePrefixReq: Boolean)(implicit ctx: Context): Type = tp
   override def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = ()
   override def checkFeasibleParent(tp: Type, pos: Position, where: => String = "")(implicit ctx: Context): Type = tp
-  override def checkInlineConformant(tree: Tree, what: => String)(implicit ctx: Context) = ()
+  override def checkInlineConformant(tree: Tree, isFinal: Boolean, what: => String)(implicit ctx: Context) = ()
   override def checkNoDoubleDeclaration(cls: Symbol)(implicit ctx: Context): Unit = ()
   override def checkParentCall(call: Tree, caller: ClassSymbol)(implicit ctx: Context) = ()
   override def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree = tpt
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 431721dc44e6..fcba55cf99d4 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1371,7 +1371,7 @@ class Typer extends Namer
     }
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
     if (sym.is(Inline, butNot = DeferredOrTermParamOrAccessor))
-      checkInlineConformant(rhs1, em"right-hand side of inline $sym")
+      checkInlineConformant(rhs1, isFinal = sym.is(Final), em"right-hand side of inline $sym")
     patchIfLazy(vdef1)
     patchFinalVals(vdef1)
     vdef1
@@ -1392,7 +1392,7 @@ class Typer extends Namer
    *  and instead return the value. This seemingly minor optimization has huge effect on initialization
    *  order and the values that can be observed during superconstructor call
    *
-   *  see remark about idempotency in PostTyper#normalizeTree
+   *  see remark about idempotency in TreeInfo#constToLiteral
    */
   private def patchFinalVals(vdef: ValDef)(implicit ctx: Context): Unit = {
     def isFinalInlinableVal(sym: Symbol): Boolean = {
@@ -2329,7 +2329,7 @@ class Typer extends Namer
       }
       else if (tree.tpe <:< pt) {
         if (pt.hasAnnotation(defn.InlineParamAnnot))
-          checkInlineConformant(tree, "argument to inline parameter")
+          checkInlineConformant(tree, isFinal = false, "argument to inline parameter")
         if (Inliner.hasBodyToInline(tree.symbol) &&
             !ctx.owner.ownersIterator.exists(_.isInlineMethod) &&
             !ctx.settings.YnoInline.value &&
diff --git a/compiler/test/dotty/tools/vulpix/TestConfiguration.scala b/compiler/test/dotty/tools/vulpix/TestConfiguration.scala
index 845908be2e0f..486d968a24bb 100644
--- a/compiler/test/dotty/tools/vulpix/TestConfiguration.scala
+++ b/compiler/test/dotty/tools/vulpix/TestConfiguration.scala
@@ -45,7 +45,7 @@ object TestConfiguration {
   }
 
   // Ideally should be Ycheck:all
-  val yCheckOptions = Array("-Ycheck:elimJavaPackages,refchecks,splitter,arrayConstructors,erasure,capturedVars,getClass,elimStaticThis,labelDef")
+  val yCheckOptions = Array("-Ycheck:all")
 
   val basicDefaultOptions = checkOptions ++ noCheckOptions ++ yCheckOptions
   val defaultUnoptimised = TestFlags(classPath, runClassPath, basicDefaultOptions)
diff --git a/tests/pos/i3971.scala b/tests/pos/i3971.scala
new file mode 100644
index 000000000000..d89d9c1b36f9
--- /dev/null
+++ b/tests/pos/i3971.scala
@@ -0,0 +1,3 @@
+object BadInlineConstCheck {
+  final val MaxSize = Int.MaxValue + 0
+}