Fix constToLiteral

We previously converted an expression with constant type to a literal if
the expression was idempotent. This can hide side effects in the case where
the expression is a selection from an object or lazy val. Demanding purity
instead prodcues tons of errors involving inline vals on objects.

We now demand idempotency if the expression refers to an inline val (or
an operation over an inline val), and purity elsewhere.

Fixes #2266

Author: odersky

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

@@ -577,21 +577,28 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
         // blocks returning a class literal alone, even if they're idempotent.
         tree1
       case ConstantType(value) =>
-        if (isIdempotentExpr(tree1)) Literal(value).withSpan(tree.span)
-        else {
-          def keepPrefix(pre: Tree) =
+        def dropOp(t: Tree): Tree = t match
+          case Select(pre, _) if t.tpe.isInstanceOf[ConstantType] =>
+            // it's a primitive unary operator
+            pre
+          case Apply(TypeApply(Select(pre, nme.getClass_), _), Nil) =>
+            pre
+          case _ =>
+            tree1
+            
+        val countsAsPure =
+          if dropOp(tree1).symbol.isInlineVal
+          then isIdempotentExpr(tree1)
+          else isPureExpr(tree1)
+          
+        if countsAsPure then Literal(value).withSpan(tree.span)
+        else
+          val pre = dropOp(tree1)
+          if pre eq tree1 then tree1
+          else
+            // it's a primitive unary operator or getClass call;
+            // Simplify `pre.op` to `{ pre; v }` where `v` is the value of `pre.op`
             Block(pre :: Nil, Literal(value)).withSpan(tree.span)
-
-          tree1 match {
-            case Select(pre, _) if tree1.tpe.isInstanceOf[ConstantType] =>
-              // it's a primitive unary operator; Simplify `pre.op` to `{ pre; v }` where `v` is the value of `pre.op`
-              keepPrefix(pre)
-            case Apply(TypeApply(Select(pre, nme.getClass_), _), Nil) =>
-              keepPrefix(pre)
-            case _ =>
-              tree1
-          }
-        }
       case _ => tree1
     }
   }

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -110,6 +110,11 @@ object SymUtils:
           self.isCoDefinedGiven(res.typeSymbol)
       self.isAllOf(Given | Method) && isCodefined(self.info)
 
+    // TODO Scala 3.x: only check for inline vals (no final ones)
+    def isInlineVal(using Context) =
+      self.isOneOf(FinalOrInline, butNot = Mutable)
+      && (!self.is(Method) || self.is(Accessor))
+
     def useCompanionAsSumMirror(using Context): Boolean =
       def companionExtendsSum(using Context): Boolean =
         self.linkedClass.isSubClass(defn.Mirror_SumClass)

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

@@ -1694,11 +1694,6 @@ class Namer { typer: Typer =>
         case _ =>
           approxTp
 
-    // println(s"final inherited for $sym: ${inherited.toString}") !!!
-    // println(s"owner = ${sym.owner}, decls = ${sym.owner.info.decls.show}")
-    // TODO Scala 3.1: only check for inline vals (no final ones)
-    def isInlineVal = sym.isOneOf(FinalOrInline, butNot = Method | Mutable)
-
     var rhsCtx = ctx.fresh.addMode(Mode.InferringReturnType)
     if sym.isInlineMethod then rhsCtx = rhsCtx.addMode(Mode.InlineableBody)
     if sym.is(ExtensionMethod) then rhsCtx = rhsCtx.addMode(Mode.InExtensionMethod)
@@ -1732,7 +1727,7 @@ class Namer { typer: Typer =>
         // don't strip @uncheckedVariance annot for default getters
         TypeOps.simplify(tp.widenTermRefExpr,
             if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null) match
-          case ctp: ConstantType if isInlineVal => ctp
+          case ctp: ConstantType if sym.isInlineVal => ctp
           case tp => TypeComparer.widenInferred(tp, pt)
 
     // Replace aliases to Unit by Unit itself. If we leave the alias in
@@ -1743,7 +1738,7 @@ class Namer { typer: Typer =>
     def lhsType = fullyDefinedType(cookedRhsType, "right-hand side", mdef.span)
     //if (sym.name.toString == "y") println(i"rhs = $rhsType, cooked = $cookedRhsType")
     if (inherited.exists)
-      if (isInlineVal) lhsType else inherited
+      if sym.isInlineVal then lhsType else inherited
     else {
       if (sym.is(Implicit))
         mdef match {

tests/pos/java-annot/S.scala

@@ -1,5 +1,5 @@
 object C {
-  val cs: "cs" = "cs"
+  final val cs: "cs" = "cs"
 }
 
 object S {

tests/run/i2266.check

@@ -0,0 +1,3 @@
+X
+true
+1

tests/run/i2266.scala

@@ -1,8 +1,13 @@
 object Test extends App {
-  def f = {
-    lazy val x: true = { println("X"); true }
-    println(x)
+  lazy val x: true = { println("X"); true }
+  println(x)
+
+  object Inner {
+    println("Y")  // not printed
+    inline val y = 1
   }
-  f
+  println(Inner.y)
+
+  inline val MV = Int.MaxValue
 }