Fix #3984: SAM closures returning Unit may need adaptation

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -984,28 +984,39 @@ class Definitions {
     isNonDepFunctionType(tp.dropDependentRefinement)
 
   // Specialized type parameters defined for scala.Function{0,1,2}.
-  private lazy val Function1SpecializedParams: collection.Set[Type] =
+  lazy val Function1SpecializedParamTypes: collection.Set[TypeRef] =
     Set(IntType, LongType, FloatType, DoubleType)
-  private lazy val Function2SpecializedParams: collection.Set[Type] =
+  lazy val Function2SpecializedParamTypes: collection.Set[TypeRef] =
     Set(IntType, LongType, DoubleType)
-  private lazy val Function0SpecializedReturns: collection.Set[Type] =
-    ScalaNumericValueTypeList.toSet[Type] + UnitType + BooleanType
-  private lazy val Function1SpecializedReturns: collection.Set[Type] =
+  lazy val Function0SpecializedReturnTypes: collection.Set[TypeRef] =
+    ScalaNumericValueTypeList.toSet + UnitType + BooleanType
+  lazy val Function1SpecializedReturnTypes: collection.Set[TypeRef] =
     Set(UnitType, BooleanType, IntType, FloatType, LongType, DoubleType)
-  private lazy val Function2SpecializedReturns: collection.Set[Type] =
-    Function1SpecializedReturns
+  lazy val Function2SpecializedReturnTypes: collection.Set[TypeRef] =
+    Function1SpecializedReturnTypes
+
+  lazy val Function1SpecializedParamClasses =
+    new PerRun[collection.Set[Symbol]](implicit ctx => Function1SpecializedParamTypes.map(_.symbol))
+  lazy val Function2SpecializedParamClasses =
+    new PerRun[collection.Set[Symbol]](implicit ctx => Function2SpecializedParamTypes.map(_.symbol))
+  lazy val Function0SpecializedReturnClasses =
+    new PerRun[collection.Set[Symbol]](implicit ctx => Function0SpecializedReturnTypes.map(_.symbol))
+  lazy val Function1SpecializedReturnClasses =
+    new PerRun[collection.Set[Symbol]](implicit ctx => Function1SpecializedReturnTypes.map(_.symbol))
+  lazy val Function2SpecializedReturnClasses =
+    new PerRun[collection.Set[Symbol]](implicit ctx => Function2SpecializedReturnTypes.map(_.symbol))
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(implicit ctx: Context) =
-    isFunctionClass(cls) && (paramTypes match {
+    paramTypes.length <= 2 && cls.derivesFrom(FunctionClass(paramTypes.length)) && (paramTypes match {
       case Nil =>
-        Function0SpecializedReturns.contains(retType)
+        Function0SpecializedReturnClasses().contains(retType.typeSymbol)
       case List(paramType0) =>
-        Function1SpecializedParams.contains(paramType0) &&
-        Function1SpecializedReturns.contains(retType)
+        Function1SpecializedParamClasses().contains(paramType0.typeSymbol) &&
+        Function1SpecializedReturnClasses().contains(retType.typeSymbol)
       case List(paramType0, paramType1) =>
-        Function2SpecializedParams.contains(paramType0) &&
-        Function2SpecializedParams.contains(paramType1) &&
-        Function2SpecializedReturns.contains(retType)
+        Function2SpecializedParamClasses().contains(paramType0.typeSymbol) &&
+        Function2SpecializedParamClasses().contains(paramType1.typeSymbol) &&
+        Function2SpecializedReturnClasses().contains(retType.typeSymbol)
       case _ =>
         false
     })

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

@@ -620,25 +620,37 @@ object Erasure {
           //     def $anonfun1(x: Object): Object = $anonfun(BoxesRunTime.unboxToInt(x))
           //     val f: Function1 = closure($anonfun1)
           //
-          // In general, a bridge is needed when, after Erasure:
-          // - one of the parameter type of the closure method is a non-reference type,
-          //   and the corresponding type in the SAM is a reference type
-          // - or the result type of the closure method is an erased value type
-          //   and the result type in the SAM isn't
-          // However, the following exception exists: If the SAM is replaced by
-          // JFunction*mc* in [[FunctionalInterfaces]], no bridge is needed: the
-          // SAM contains default methods to handle adaptation
+          // In general a bridge is needed when, after Erasure, one of the
+          // parameter type or the result type of the closure method has a
+          // different type, and we cannot rely on auto-adaptation.
+          //
+          // Auto-adaptation works in the following cases:
+          // - If the SAM is replaced by JFunction*mc* in
+          //   [[FunctionalInterfaces]], no bridge is needed: the SAM contains
+          //   default methods to handle adaptation.
+          // - If a result type of the closure method is a primitive value type
+          //   different from Unit, we can rely on the auto-adaptation done by
+          //   LMF (because it only needs to box, not unbox, so no special
+          //   handling of null is required).
+          // - If the SAM is replaced by JProcedure* in
+          //   [[DottyBackendInterface]] (this only happens when no explicit SAM
+          //   type is given), no bridge is needed to box a Unit result type:
+          //   the SAM contains a default method to handle that.
           //
           // See test cases lambda-*.scala and t8017/ for concrete examples.
 
-          def isReferenceType(tp: Type) = !tp.isPrimitiveValueType && !tp.isErasedValueType
-
           if (!defn.isSpecializableFunction(implClosure.tpe.widen.classSymbol.asClass, implParamTypes, implResultType)) {
+            def autoAdaptedParam(tp: Type) = !tp.isErasedValueType && !tp.isPrimitiveValueType
+            val explicitSAMType = implClosure.tpt.tpe.exists
+            def autoAdaptedResult(tp: Type) = !tp.isErasedValueType &&
+              (!explicitSAMType || tp.typeSymbol != defn.UnitClass)
+            def sameSymbol(tp1: Type, tp2: Type) = tp1.typeSymbol == tp2.typeSymbol
+
             val paramAdaptationNeeded =
               (implParamTypes, samParamTypes).zipped.exists((implType, samType) =>
-                !isReferenceType(implType) && isReferenceType(samType))
+                !sameSymbol(implType, samType) && !autoAdaptedParam(implType))
             val resultAdaptationNeeded =
-              implResultType.isErasedValueType && !samResultType.isErasedValueType
+              !sameSymbol(implResultType, samResultType) && !autoAdaptedResult(implResultType)
 
             if (paramAdaptationNeeded || resultAdaptationNeeded) {
               val bridgeType =

tests/run/lambda-unit.scala

@@ -2,13 +2,19 @@ trait SAMUnit {
   def foo(a: Object): Unit
 }
 
+trait GenericSAM[R] {
+  def foo(a: Object): R
+}
+
 object Test {
   val fun: Object => Unit = a => assert(a == "")
   val sam: SAMUnit = a => assert(a == "")
+  val genericSam: GenericSAM[Unit] = a => assert(a == "")
 
   def main(args: Array[String]): Unit = {
     fun("")
     (fun: Object => Any)("")
     sam.foo("")
+    genericSam.foo("")
   }
 }