Move the logic of ExpandSAMs.isJvmSam to Platform.isSam.

Whether a language SAM type is also a valid SAM type for the
back-end is a platform-specific thing. On Scala.js, for example,
the rules are completely different than for the JVM.

This commit therefore moves the logic of the predicate used by
ExpandSAMs to decide whether to expand a SAM as an anonymous class
to the Platform.

Author: sjrd

src/dotty/tools/dotc/config/JavaPlatform.scala

@@ -7,6 +7,7 @@ import ClassPath.{ JavaContext, DefaultJavaContext }
 import core._
 import Symbols._, Types._, Contexts._, Denotations._, SymDenotations._, StdNames._, Names._
 import Flags._, Scopes._, Decorators._, NameOps._, util.Positions._
+import transform.ExplicitOuter, transform.SymUtils._
 
 class JavaPlatform extends Platform {
 
@@ -38,6 +39,14 @@ class JavaPlatform extends Platform {
 
   def rootLoader(root: TermSymbol)(implicit ctx: Context): SymbolLoader = new ctx.base.loaders.PackageLoader(root, classPath)
 
+  /** Is the SAMType `cls` also a SAM under the rules of the JVM? */
+  def isSam(cls: ClassSymbol)(implicit ctx: Context): Boolean =
+    cls.is(NoInitsTrait) &&
+    cls.superClass == defn.ObjectClass &&
+    cls.directlyInheritedTraits.forall(_.is(NoInits)) &&
+    !ExplicitOuter.needsOuterIfReferenced(cls) &&
+    cls.typeRef.fields.isEmpty // Superaccessors already show up as abstract methods here, so no test necessary
+
   /** We could get away with excluding BoxedBooleanClass for the
    *  purpose of equality testing since it need not compare equal
    *  to anything but other booleans, but it should be present in

src/dotty/tools/dotc/config/Platform.scala

@@ -27,6 +27,9 @@ abstract class Platform {
   /** Any platform-specific phases. */
   //def platformPhases: List[SubComponent]
 
+  /** Is the SAMType `cls` also a SAM under the rules of the platform? */
+  def isSam(cls: ClassSymbol)(implicit ctx: Context): Boolean
+
   /** The various ways a boxed primitive might materialize at runtime. */
   def isMaybeBoxed(sym: ClassSymbol)(implicit ctx: Context): Boolean
 

src/dotty/tools/dotc/transform/ExpandSAMs.scala

@@ -25,21 +25,17 @@ class ExpandSAMs extends MiniPhaseTransform { thisTransformer =>
 
   import ast.tpd._
 
-  /** Is SAMType `cls` also a SAM under the rules of the JVM? */
-  def isJvmSam(cls: ClassSymbol)(implicit ctx: Context): Boolean =
-    cls.is(NoInitsTrait) &&
-    cls.superClass == defn.ObjectClass &&
-    cls.directlyInheritedTraits.forall(_.is(NoInits)) &&
-    !ExplicitOuter.needsOuterIfReferenced(cls) &&
-    cls.typeRef.fields.isEmpty // Superaccessors already show up as abstract methods here, so no test necessary
+  /** Is the SAMType `cls` also a SAM under the rules of the platform? */
+  def isPlatformSam(cls: ClassSymbol)(implicit ctx: Context): Boolean =
+    ctx.platform.isSam(cls)
 
   override def transformBlock(tree: Block)(implicit ctx: Context, info: TransformerInfo): Tree = tree match {
     case Block(stats @ (fn: DefDef) :: Nil, Closure(_, fnRef, tpt)) if fnRef.symbol == fn.symbol =>
       tpt.tpe match {
         case NoType => tree // it's a plain function
         case tpe @ SAMType(_) if tpe.isRef(defn.PartialFunctionClass) =>
           toPartialFunction(tree)
-        case tpe @ SAMType(_) if isJvmSam(tpe.classSymbol.asClass) =>
+        case tpe @ SAMType(_) if isPlatformSam(tpe.classSymbol.asClass) =>
           tree
         case tpe =>
           cpy.Block(tree)(stats,