Align SAM test and expansion

Fix SAM test to use the same scheme as SAM expansion to determine whether
a type needs zero arguments for construction.

Author: odersky

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

@@ -328,21 +328,6 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       superArgs: List[Tree] = Nil, adaptVarargs: Boolean = false)(using Context): TypeDef =
     val firstParent :: otherParents = cls.info.parents: @unchecked
 
-    def isApplicable(constr: Symbol): Boolean =
-      def recur(ctpe: Type): Boolean = ctpe match
-        case ctpe: PolyType =>
-          recur(ctpe.instantiate(firstParent.argTypes))
-        case ctpe: MethodType =>
-          var paramInfos = ctpe.paramInfos
-          if adaptVarargs && paramInfos.length == superArgs.length + 1
-            && atPhaseNoLater(Phases.elimRepeatedPhase)(constr.info.isVarArgsMethod)
-          then // accept missing argument for varargs parameter
-            paramInfos = paramInfos.init
-          superArgs.corresponds(paramInfos)(_.tpe <:< _)
-        case _ =>
-          false
-      recur(constr.info)
-
     def adaptedSuperArgs(ctpe: Type): List[Tree] = ctpe match
       case ctpe: PolyType =>
         adaptedSuperArgs(ctpe.instantiate(firstParent.argTypes))
@@ -357,8 +342,11 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     val superRef =
       if cls.is(Trait) then TypeTree(firstParent)
       else
-        val constr = firstParent.decl(nme.CONSTRUCTOR).suchThat(isApplicable)
-        New(firstParent, constr.symbol.asTerm, adaptedSuperArgs(constr.info))
+        val parentConstr = firstParent.applicableConstructors(superArgs.tpes, adaptVarargs) match
+          case Nil => assert(false, i"no applicable parent constructor of $firstParent for supercall arguments $superArgs")
+          case constr :: Nil => constr
+          case _ => assert(false, i"multiple applicable parent constructors of $firstParent for supercall arguments $superArgs")
+        New(firstParent, parentConstr.asTerm, adaptedSuperArgs(parentConstr.info))
 
     ClassDefWithParents(cls, constr, superRef :: otherParents.map(TypeTree(_)), body)
   end ClassDef

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

@@ -8,6 +8,7 @@ import Names.{Name, TermName}
 import Constants.Constant
 
 import Names.Name
+import StdNames.nme
 import config.Feature
 
 class TypeUtils:
@@ -189,5 +190,30 @@ class TypeUtils:
     def stripRefinement: Type = self match
       case self: RefinedOrRecType => self.parent.stripRefinement
       case seld => self
+
+    /** The constructors of this tyoe that that are applicable to `argTypes`, without needing
+     *  an implicit conversion.
+     *  @param adaptVarargs   if true, allow a constructor with just a varargs argument to
+     *                        match an empty argument list.
+     */
+    def applicableConstructors(argTypes: List[Type], adaptVarargs: Boolean)(using Context): List[Symbol] =
+      def isApplicable(constr: Symbol): Boolean =
+        def recur(ctpe: Type): Boolean = ctpe match
+          case ctpe: PolyType =>
+            if argTypes.isEmpty then recur(ctpe.resultType) // no need to know instances
+            else recur(ctpe.instantiate(self.argTypes))
+          case ctpe: MethodType =>
+            var paramInfos = ctpe.paramInfos
+            if adaptVarargs && paramInfos.length == argTypes.length + 1
+              && atPhaseNoLater(Phases.elimRepeatedPhase)(constr.info.isVarArgsMethod)
+            then // accept missing argument for varargs parameter
+              paramInfos = paramInfos.init
+            argTypes.corresponds(paramInfos)(_ <:< _)
+          case _ =>
+            false
+        recur(constr.info)
+
+      self.decl(nme.CONSTRUCTOR).altsWith(isApplicable).map(_.symbol)
+
 end TypeUtils
 

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

@@ -5944,22 +5944,18 @@ object Types extends TypeUtils {
 
     def samClass(tp: Type)(using Context): Symbol = tp match
       case tp: ClassInfo =>
-        def zeroParams(tp: Type): Boolean = tp.stripPoly match
-          case mt: MethodType =>
-            val noArgsNeeded = mt.paramInfos match
-              case Nil => true
-              case info :: Nil => info.isRepeatedParam
-              case _ => false
-            noArgsNeeded && !mt.resultType.isInstanceOf[MethodType]
-          case et: ExprType => true
-          case _ => false
-        def validCtor(cls: Symbol): Boolean =
-          val ctor = cls.primaryConstructor
-          (!ctor.exists || zeroParams(ctor.info)) // `ContextFunctionN` does not have constructors
-          && (!cls.is(Trait) || validCtor(cls.info.parents.head.classSymbol))
+        val cls = tp.cls
+        def takesNoArgs(tp: Type) =
+          !tp.classSymbol.primaryConstructor.exists
+              // e.g. `ContextFunctionN` does not have constructors
+          || tp.applicableConstructors(Nil, adaptVarargs = true).lengthCompare(1) == 0
+              // we require a unique constructor so that SAM expansion is deterministic
+        val noArgsNeeded: Boolean =
+          takesNoArgs(tp)
+          && (!tp.cls.is(Trait) || takesNoArgs(tp.parents.head))
         def isInstantiable =
           !tp.cls.isOneOf(FinalOrSealed) && (tp.appliedRef <:< tp.selfType)
-        if validCtor(tp.cls) && isInstantiable then tp.cls
+        if noArgsNeeded && isInstantiable then tp.cls
         else NoSymbol
       case tp: AppliedType =>
         samClass(tp.superType)