Fix #1877: Add forwarders for primitive/generic mixins.

Author: nicolasstucki

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

@@ -48,11 +48,7 @@ class MixinOps(cls: ClassSymbol, thisTransform: DenotTransformer)(implicit ctx:
    *   - there are multiple traits defining method with same signature
    */
   def needsForwarder(meth: Symbol): Boolean = {
-    lazy val competingMethods = cls.baseClasses.iterator
-      .filter(_ ne meth.owner)
-      .map(meth.overriddenSymbol)
-      .filter(_.exists)
-      .toList
+    lazy val competingMethods = competingMethodsIterator(meth).toList
 
     def needsDisambiguation = competingMethods.exists(x=> !(x is Deferred)) // multiple implementations are available
     def hasNonInterfaceDefinition = competingMethods.exists(!_.owner.is(Trait)) // there is a definition originating from class
@@ -61,8 +57,34 @@ class MixinOps(cls: ClassSymbol, thisTransform: DenotTransformer)(implicit ctx:
     (needsDisambiguation || hasNonInterfaceDefinition || meth.owner.is(Scala2x))
   }
 
+  /** Get `sym` of the method that needs a forwarder
+    *  Method needs a forwarder in those cases:
+    *   - there is a trait that defines a primitive version of implemented polymorphic method.
+    *   - there is a trait that defines a polymorphic version of implemented primitive method.
+    */
+  def needsPrimitiveForwarderTo(meth: Symbol): Option[Symbol] = {
+    def hasPrimitiveMissMatch(tp1: Type, tp2: Type): Boolean = (tp1, tp2) match {
+      case (tp1: MethodicType, tp2: MethodicType) => hasPrimitiveMissMatch(tp1.resultType, tp2.resultType)
+      case _ => tp1.typeSymbol.isPrimitiveValueClass ^ tp2.typeSymbol.isPrimitiveValueClass
+    }
+
+    def needsPrimitiveForwarder(m: Symbol): Boolean =
+      m.owner != cls && !m.is(Deferred) && hasPrimitiveMissMatch(meth.info, m.info)
+
+    if (!meth.is(Method | Deferred, butNot = PrivateOrAccessor) || meth.overriddenSymbol(cls).exists || needsForwarder(meth)) None
+    else competingMethodsIterator(meth).find(needsPrimitiveForwarder)
+  }
+
+  final val PrivateOrAccessor = Private | Accessor
   final val PrivateOrAccessorOrDeferred = Private | Accessor | Deferred
 
   def forwarder(target: Symbol) = (targs: List[Type]) => (vrefss: List[List[Tree]]) =>
     superRef(target).appliedToTypes(targs).appliedToArgss(vrefss)
+
+  private def competingMethodsIterator(meth: Symbol): Iterator[Symbol] = {
+    cls.baseClasses.iterator
+      .filter(_ ne meth.owner)
+      .map(meth.overriddenSymbol)
+      .filter(_.exists)
+  }
 }

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

@@ -38,6 +38,14 @@ import ResolveSuper._
  *
  *                <mods> def f[Ts](ps1)...(psN): U = super[M].f[Ts](ps1)...(psN)
  *
+ *          3.3 (done in `methodPrimitiveForwarders`) For every method that is declared both
+ *              as generic with a primitive type and with a primitive type
+ *              `<mods> def f[Ts](ps1)...(psN): U` in trait M` and
+ *              `<mods> def f[Ts](ps1)...(psN): V = ...` in implemented in N`
+ *              where U is a primitive and V a polymorphic type (or vice versa) needs:
+ *
+ *                <mods> def f[Ts](ps1)...(psN): U = super[N].f[Ts](ps1)...(psN)
+ *
  *        A method in M needs to be disambiguated if it is concrete, not overridden in C,
  *        and if it overrides another concrete method.
  *
@@ -65,7 +73,11 @@ class ResolveSuper extends MiniPhaseTransform with IdentityDenotTransformer { th
       for (meth <- mixin.info.decls.toList if needsForwarder(meth))
         yield polyDefDef(implementation(meth.asTerm), forwarder(meth))
 
-    val overrides = mixins.flatMap(mixin => superAccessors(mixin) ::: methodOverrides(mixin))
+    def methodPrimitiveForwarders: List[Tree] =
+      for (meth <- mixins.flatMap(_.info.decls.flatMap(needsPrimitiveForwarderTo)).distinct)
+        yield polyDefDef(implementation(meth.asTerm), forwarder(meth))
+
+    val overrides = mixins.flatMap(mixin => superAccessors(mixin) ::: methodOverrides(mixin)) ::: methodPrimitiveForwarders
 
     cpy.Template(impl)(body = overrides ::: impl.body)
   }

tests/run/mixin-primitive-on-generic-1.check

@@ -0,0 +1,2 @@
+true
+true

tests/run/mixin-primitive-on-generic-1.scala

@@ -0,0 +1,19 @@
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println((new Foo: Baz).value1)
+    println((new Foo: Baz).value2)
+  }
+}
+
+class Foo extends Bar[Boolean](true) with Baz
+
+class Bar[T](x: T) {
+  def value1: T = x
+  def value2(): T = x
+}
+
+trait Baz {
+  def value1: Boolean
+  def value2(): Boolean
+}

tests/run/mixin-primitive-on-generic-2.check

@@ -0,0 +1,2 @@
+true
+true

tests/run/mixin-primitive-on-generic-2.scala

@@ -0,0 +1,19 @@
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println((new Foo: Bar[Boolean]).value1)
+    println((new Foo: Bar[Boolean]).value2)
+  }
+}
+
+class Foo extends Baz with Bar[Boolean]
+
+trait Bar[T] {
+  def value1: T
+  def value2(): T
+}
+
+class Baz {
+  def value1: Boolean = true
+  def value2(): Boolean = true
+}

tests/run/mixin-primitive-on-generic-3.check

@@ -0,0 +1,2 @@
+true
+true

tests/run/mixin-primitive-on-generic-3.scala

@@ -0,0 +1,21 @@
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println((new Foo: Baz).value)
+    println((new Foo: Qux).value)
+  }
+}
+
+class Foo extends Bar[Boolean](true) with Baz with Qux
+
+class Bar[T](x: T) {
+  def value: T = x
+}
+
+trait Baz {
+  def value: Boolean
+}
+
+trait Qux {
+  def value: Boolean
+}