Fix #8602: Do not create mixins for constant-expression final vals.

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

@@ -208,7 +208,7 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
             } else
               dropped += sym
           case stat @ DefDef(name, _, _, tpt, _)
-              if stat.symbol.isGetter && stat.symbol.owner.is(Trait) && !stat.symbol.is(Lazy) =>
+              if stat.symbol.isGetter && stat.symbol.owner.is(Trait) && !stat.symbol.is(Lazy) && !stat.symbol.isConstExprFinalVal =>
             val sym = stat.symbol
             assert(isRetained(sym), sym)
             if !stat.rhs.isEmpty && !isWildcardArg(stat.rhs) then

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

@@ -126,7 +126,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
 
     if (sym.is(Accessor, butNot = Deferred) && ownerIsTrait) {
       val sym1 =
-        if (sym.is(Lazy)) sym
+        if (sym.is(Lazy) || sym.symbol.isConstExprFinalVal) sym
         else sym.copySymDenotation(initFlags = sym.flags &~ (ParamAccessor | Inline) | Deferred)
       sym1.ensureNotPrivate
     }
@@ -166,7 +166,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
   private def needsTraitSetter(sym: Symbol)(using Context): Boolean =
     sym.isGetter && !wasOneOf(sym, DeferredOrLazy | ParamAccessor)
       && atPhase(thisPhase) { !sym.setter.exists }
-      && !sym.info.resultType.isInstanceOf[ConstantType]
+      && !sym.isConstExprFinalVal
 
   private def makeTraitSetter(getter: TermSymbol)(using Context): Symbol =
     getter.copy(
@@ -251,7 +251,10 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
               cls.srcPos)
           EmptyTree
 
-      for (getter <- mixin.info.decls.toList if getter.isGetter && !wasOneOf(getter, Deferred)) yield {
+      for
+        getter <- mixin.info.decls.toList
+        if getter.isGetter && !wasOneOf(getter, Deferred) && !getter.isConstExprFinalVal
+      yield
         if (isCurrent(getter) || getter.name.is(ExpandedName)) {
           val rhs =
             if (wasOneOf(getter, ParamAccessor))
@@ -266,7 +269,6 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
           transformFollowing(DefDef(mkForwarderSym(getter.asTerm), rhs))
         }
         else EmptyTree
-      }
     }
 
     def setters(mixin: ClassSymbol): List[Tree] =

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

@@ -16,6 +16,7 @@ import ValueClasses.isDerivedValueClass
 import Decorators._
 import Constants.Constant
 import Annotations.Annotation
+import Phases._
 import ast.tpd.Literal
 
 import language.implicitConversions
@@ -167,6 +168,11 @@ object SymUtils:
       self.owner.info.decl(fieldName).suchThat(!_.is(Method)).symbol
     }
 
+    def isConstExprFinalVal(using Context): Boolean =
+      atPhaseNoLater(erasurePhase) {
+        self.is(Final) && self.info.resultType.isInstanceOf[ConstantType]
+      }
+
     def isField(using Context): Boolean =
       self.isTerm && !self.is(Method)
 
@@ -268,4 +274,4 @@ object SymUtils:
           Annotation(defn.TargetNameAnnot,
             Literal(Constant(nameFn(original.targetName).toString)).withSpan(original.span)))
   end extension
-end SymUtils
+end SymUtils

tests/run/i8602/ChildClass_2.java

@@ -0,0 +1,5 @@
+public class ChildClass_2 extends Object implements ParentTrait {
+  public int getFoo() {
+    return foo();
+  }
+}

tests/run/i8602/ParentTrait_1.scala

@@ -0,0 +1,3 @@
+trait ParentTrait {
+  final val foo = 5
+}

tests/run/i8602/Test_3.scala

@@ -0,0 +1,8 @@
+object Test {
+  def main(args: Array[String]): Unit =
+    val child = new ChildClass_2()
+    assert(child.foo == 5)
+    assert(child.getFoo() == 5)
+    val parent: ParentTrait = child
+    assert(parent.foo == 5)
+}