Revise SAM test

Author: odersky

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

@@ -5963,25 +5963,21 @@ object Types extends TypeUtils {
             noArgsNeeded && !mt.resultType.isInstanceOf[MethodType]
           case et: ExprType => true
           case _ => false
-        def validCtor(cls: Symbol): Boolean =
-          val ctor = cls.primaryConstructor
-          if !ctor.exists then true // `ContextFunctionN` does not have constructors
-          else if !zeroParams(ctor.info) then false
-          else if !cls.is(Trait) then true
-          else
-            val firstParentCls = cls.info.parents.head.classSymbol
-            if firstParentCls.isClass && !firstParentCls.is(Trait) then
-              validCtor(firstParentCls) // need to check class constructor as well
-            else
-              true
-        val validCtorNEW = validCtor(tp.cls)
-
-        if validCtorNEW != validCtorOLD then
-          println(i"SAM change for $tp with parent ${cls.info.parents.head.classSymbol.fullName}, now $validCtorNEW")
+        def takesNoArgs(tp: Type) =
+          val constrs = tp.decl(nme.CONSTRUCTOR)
+          !constrs.exists // `ContextFunctionN` does not have constructors
+          || constrs.hasAltWith(constr => zeroParams(constr.info))
+        def firstParentCls = cls.info.parents.head.classSymbol
+        val noArgsNeeded: Boolean =
+          takesNoArgs(tp)
+          && (!tp.cls.is(Trait) || takesNoArgs(tp.cls.info.parents.head))
+
+        if noArgsNeeded != validCtorOLD then
+          println(i"SAM change for $tp with parent ${firstParentCls.fullName}, now $noArgsNeeded")
 
         def isInstantiable =
           !tp.cls.isOneOf(FinalOrSealed) && (tp.appliedRef <:< tp.selfType)
-        if validCtorNEW && isInstantiable then tp.cls
+        if noArgsNeeded && isInstantiable then tp.cls
         else NoSymbol
       case tp: AppliedType =>
         samClass(tp.superType)