Don't treat SAM class with Unit result type as a platform SAM

Author: noti0na1

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

@@ -50,7 +50,10 @@ class JavaPlatform extends Platform {
     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
+    // Superaccessors already show up as abstract methods here, so no test necessary
+    cls.typeRef.fields.isEmpty &&
+    // LambdaMetafactory can't handle SAMs with Unit return type unless it's a FunctionN itself
+    !cls.typeRef.possibleSamMethods.exists(_.info.resultType.isRef(defn.UnitClass))
 
   /** We could get away with excluding BoxedBooleanClass for the
    *  purpose of equality testing since it need not compare equal

tests/run/i24573.check

@@ -0,0 +1,5 @@
+1
+2
+3
+42
+hello

tests/run/i24573.scala

@@ -0,0 +1,25 @@
+trait Con[-T] extends (T => Unit):
+  def apply(t: T): Unit
+
+trait Con2[-T] extends (T => Int):
+  def apply(t: T): Int
+
+trait Con3[+R] extends (() => R):
+  def apply(): R
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val f1: (Int => Unit) = i => println(i)
+    f1(1)
+
+    val c1: Con[Int] = i => println(i)
+    c1(2)
+
+    val c2: Con2[Int] = i => { println(i); i }
+    c2(3)
+
+    val c3: Con3[Int] = () => 42
+    println(c3())
+
+    val c4: Con3[Unit] = () => println("hello")
+    c4()