Fix initializing parents of AnonClasses

Previously creating ClassDefs of AnonClasses was only possible if the
parent class had a constructor wih an empty parameter list. Otherwise
the compiler would crash becasue of calling `asTerm` on a `NoSymbol`.
This PR adds logic that handles callig parent constructors with default
or repeated parameters. And also fails more gracefuly if no instance can
be created.

fixes lampepfl#15855

Author: KacperFKorban

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -292,23 +292,58 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def TypeDef(sym: TypeSymbol)(using Context): TypeDef =
     ta.assignType(untpd.TypeDef(sym.name, TypeTree(sym.info)), sym)
 
-  def ClassDef(cls: ClassSymbol, constr: DefDef, body: List[Tree], superArgs: List[Tree] = Nil)(using Context): TypeDef = {
+  def ClassDef(cls: ClassSymbol, constrDef: DefDef, body: List[Tree])(using Context): TypeDef = {
     val firstParent :: otherParents = cls.info.parents: @unchecked
     val superRef =
       if (cls.is(Trait)) TypeTree(firstParent)
       else {
-        def isApplicable(ctpe: Type): Boolean = ctpe match {
-          case ctpe: PolyType =>
-            isApplicable(ctpe.instantiate(firstParent.argTypes))
-          case ctpe: MethodType =>
-            (superArgs corresponds ctpe.paramInfos)(_.tpe <:< _)
+        def fromRepeated(tpe: Type): Type = tpe match {
+          case AnnotatedType(AppliedType(base, List(tparam)), annot) if annot.symbol == defn.RepeatedAnnot =>
+            tparam
           case _ =>
-            false
+            NoType
         }
-        val constr = firstParent.decl(nme.CONSTRUCTOR).suchThat(constr => isApplicable(constr.info))
-        New(firstParent, constr.symbol.asTerm, superArgs)
+
+        def canConstructMethod(constrSym: Symbol): Boolean = constrSym.info match {
+          case ctpe: MethodOrPoly =>
+            constrSym.paramSymss.flatten.filter(_.isTerm).forall { paramSym =>
+              paramSym.flags.is(Flags.HasDefault) || (fromRepeated(paramSym.info) != NoType)
+            }
+          case _ => false
+        }
+
+        def constructParent(constr: Tree): Tree = {
+          constr.symbol.paramSymss.filter(!_.exists(_.isType)).foldLeft(constr) { (acc, paramList) =>
+            acc.appliedToTermArgs(paramList.zipWithIndex.collect {
+              case (paramSym, idx) if paramSym.flags.is(Flags.HasDefault) =>
+                typer.Applications.defaultArgument(acc, idx, false)
+              case (paramSym, idx) if fromRepeated(paramSym.info) != NoType =>
+                tpd.ref(defn.NilModule)
+            })
+          }
+        }
+
+        val tycon = firstParent.typeConstructor
+        val newTree = New(tycon)
+          .withSpan(cls.span)
+        val constr = firstParent
+          .decl(nme.CONSTRUCTOR)
+          .suchThat{ constr =>
+            canConstructMethod(constr)
+          }
+
+        if constr.exists then
+          constructParent(
+            newTree
+              .select(TermRef(tycon, constr.symbol.asTerm))
+              .appliedToTypes(firstParent.argTypes)
+          )
+        else
+          val msg = em"not enough arguments for constructor ${firstParent.dealias.typeSymbol.primaryConstructor.info}"
+          report.error(msg, cls.sourcePos)
+          EmptyTree
       }
-    ClassDefWithParents(cls, constr, superRef :: otherParents.map(TypeTree(_)), body)
+    ClassDefWithParents(cls, constrDef, superRef :: otherParents.map(TypeTree(_)), body)
   }
 
   def ClassDefWithParents(cls: ClassSymbol, constr: DefDef, parents: List[Tree], body: List[Tree])(using Context): TypeDef = {

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

@@ -182,4 +182,5 @@ class ExpandSAMs extends MiniPhase:
     case tpe =>
       tpe
   }
+
 end ExpandSAMs

tests/neg/i15855.scala

@@ -0,0 +1,7 @@
+class MyFunction(arg: String)
+
+trait MyFun[+R] extends MyFunction {
+  def apply(i: Int): R
+}
+
+val myFun: MyFun[Int] = (i: Int) => 1 // error

tests/run/i15855.check

@@ -0,0 +1,11 @@
+default
+List()
+other
+asdf
+420
+21.37
+1
+2
+3
+4
+5

tests/run/i15855.scala

@@ -0,0 +1,82 @@
+class MyFunction1()
+
+trait MyFun1[+R] extends MyFunction1 {
+  def apply(i: Int): R
+}
+
+val myFun1: MyFun1[Int] = (i: Int) => 1
+
+//
+
+class MyFunction2(arg: String = "default") {
+  println(arg)
+}
+
+trait MyFun2[+R] extends MyFunction2 {
+  def apply(i: Int): R
+}
+
+val myFun2: MyFun2[Int] = (i: Int) => 2
+
+//
+
+class MyFunction3(arg: String*) {
+  println(arg)
+}
+
+trait MyFun3[+R] extends MyFunction3 {
+  def apply(i: Int): R
+}
+
+val myFun3: MyFun3[Int] = (i: Int) => 3
+
+//
+
+class MyFunction4(arg: String) {
+  println(arg)
+  def this() = {
+    this("other")
+  }
+}
+
+trait MyFun4[+R] extends MyFunction4 {
+  def apply(i: Int): R
+}
+
+val myFun4: MyFun4[Int] = (i: Int) => 4
+
+//
+
+class MyFunction5(arg: String = "asdf")(arg1: Int = 420, arg2: Double = 21.37) {
+  println(arg)
+  println(arg1)
+  println(arg2)
+}
+
+trait MyFun5[+R] extends MyFunction5 {
+  def apply(i: Int): R
+}
+
+val myFun5: MyFun5[Int] = (i: Int) => 5
+
+//
+
+trait Equiv[T] extends Any with Serializable {
+  def equiv(x: T, y: T): Boolean
+}
+
+def reference[T <: AnyRef]: Equiv[T] = { _ eq _ }
+def universal[T]: Equiv[T] = { _ == _ }
+def fromFunction[T](cmp: (T, T) => Boolean): Equiv[T] = {
+  (x, y) => cmp(x, y)
+}
+
+//
+
+object Test extends App {
+  println(myFun1(1))
+  println(myFun2(1))
+  println(myFun3(1))
+  println(myFun4(1))
+  println(myFun5(1))
+}