Check if the parent of class has an empty constr before expanding SAM

Expanding a SAM into an anonymous class assumes that the parent class
has a constructor with an empty parameter list. Before this change, a
situation in which there was no such constructor caused a crash.

fixes scala#15855

Author: KacperFKorban

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

@@ -59,8 +59,12 @@ class ExpandSAMs extends MiniPhase:
         case tpe =>
           val tpe1 = checkRefinements(tpe.stripNull, fn)
           val Seq(samDenot) = tpe1.possibleSamMethods
-          cpy.Block(tree)(stats,
-              AnonClass(tpe1 :: Nil, fn.symbol.asTerm :: Nil, samDenot.symbol.asTerm.name :: Nil))
+          if hasNoArgConstr(tpe1) then
+            cpy.Block(tree)(stats,
+                AnonClass(tpe1 :: Nil, fn.symbol.asTerm :: Nil, samDenot.symbol.asTerm.name :: Nil))
+          else
+            report.error(em"${tpe1} cannot be instantiated with an empty constructor", tree.srcPos)
+            tree
       }
     case _ =>
       tree
@@ -182,4 +186,18 @@ class ExpandSAMs extends MiniPhase:
     case tpe =>
       tpe
   }
+
+  private def hasNoArgConstr(tpe: Type)(using Context): Boolean = {
+    def noArgs(ctpe: Type): Boolean = ctpe match {
+      case ctpe: PolyType =>
+        noArgs(ctpe.resType)
+      case ctpe: MethodType =>
+        ctpe.paramInfos.isEmpty
+      case _ =>
+        false
+    }
+    val parent :: _ = tpe.parents: @unchecked
+    val constr = parent.dealias.decl(nme.CONSTRUCTOR).suchThat(constr => noArgs(constr.info))
+    constr != SymDenotations.NoDenotation
+  }
 end ExpandSAMs

tests/neg/i15855.scala

@@ -0,0 +1,56 @@
+class MyFunction(arg: String) {
+  def a: String = arg
+}
+
+trait MyFun[+R] extends MyFunction {
+  def apply(i: Int): R
+}
+
+val myFun: MyFun[Int] = (i: Int) => 1 // error
+
+//
+
+class MyFunction1(arg: String = "") {
+  def a: String = arg
+}
+
+trait MyFun1[+R] extends MyFunction1 {
+  def apply(i: Int): R
+}
+
+val myFun1: MyFun1[Int] = (i: Int) => 1 // error
+
+//
+
+trait MyFunction2(arg: String = "") {
+  def a: String = arg
+}
+
+trait MyFun2[+R] extends MyFunction2 {
+  def apply(i: Int): R
+}
+
+val myFun2: MyFun2[Int] = (i: Int) => 1
+
+//
+
+trait MyFunction3(arg: String) {
+  def a: String = arg
+}
+
+trait MyFun3[+R] extends MyFunction3 {
+  def apply(i: Int): R
+}
+
+val myFun3: MyFun3[Int] = (i: Int) => 1
+
+//
+
+class MyFunction4() {
+}
+
+trait MyFun4[+R] extends MyFunction4 {
+  def apply(i: Int): R
+}
+
+val myFun4: MyFun4[Int] = (i: Int) => 1