Fix scala#3971: Relax checkInlineConformant for final vals

Necessary for -Ycheck:frontend to work

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -317,7 +317,7 @@ 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 +342,7 @@ 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 +397,7 @@ 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.

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

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 &&

tests/pos/i3971.scala

@@ -0,0 +1,3 @@
+object BadInlineConstCheck {
+  final val MaxSize = Int.MaxValue + 0
+}