Ban classes that incompatibly refine type params

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

@@ -1765,7 +1765,9 @@ class Definitions {
     Set[Symbol](ComparableClass, ProductClass, SerializableClass,
       // add these for now, until we had a chance to retrofit 2.13 stdlib
       // we should do a more through sweep through it then.
+      requiredClass("scala.collection.IterableFactoryDefaults"),
       requiredClass("scala.collection.SortedOps"),
+      requiredClass("scala.collection.StrictOptimizedSetOps"),
       requiredClass("scala.collection.StrictOptimizedSortedSetOps"),
       requiredClass("scala.collection.generic.DefaultSerializable"),
       requiredClass("scala.collection.generic.IsIterable"),

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

@@ -209,8 +209,9 @@ trait PatternTypeConstrainer { self: TypeComparer =>
    *  are used to infer type arguments to Unapply trees.
    *
    *  ## Invariant refinement
-   *  Essentially, we say that `D[B] extends C[B]` s.t. refines parameter `A` of `trait C[A]` invariantly if
-   *  when `c: C[T]` and `c` is instance of `D`, then necessarily `c: D[T]`. This is violated if `A` is variant:
+   *  Essentially, we say that `D[B] extends C[B]` refines parameter `A` of `trait C[A]` invariantly if
+   *  when `c: C[T]` and `c` is instance of `D`, then necessarily `c: D[T]`.
+   *  This is violated if `A` is variant and `C` is mixed in with an incompatible type argument:
    *
    *     trait C[+A]
    *     trait D[+B](val b: B) extends C[B]
@@ -224,29 +225,30 @@ trait PatternTypeConstrainer { self: TypeComparer =>
    *     }
    *
    *  It'd be unsound for us to say that `t <: T`, even though that follows from `D[t] <: C[T]`.
-   *  Note, however, that if `D` was a final class, we *could* rely on that relationship.
-   *  To support typical case classes, we also assume that this relationship holds for them and their parent traits.
-   *  This is enforced by checking that classes inheriting from case classes do not extend the parent traits of those
-   *  case classes without also appropriately extending the relevant case class
-   *  (see `RefChecks#checkCaseClassInheritanceInvariant`).
+   *  Note, however, that if `D` was a concrete class, we can rely on that relationship.
+   *  We can assume this relationship holds for them and their parent traits
+   *  by checking that classes inheriting from those classes do not mix-in any parent traits
+   *  with a type parameter that isn't the same type, a subtype, or a super type, depending on if the
+   *  trait's parameter is invariant, covariant or contravariant, respectively
+   *  (see `RefChecks#checkClassInheritanceInvariant`).
    */
   def constrainSimplePatternType(patternTp: Type, scrutineeTp: Type, forceInvariantRefinement: Boolean): Boolean = {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
       case tp: SingletonType => true
-      case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
+      case tp: ClassInfo => tp.cls.is(Final)
       case tp: TypeProxy => refinementIsInvariant(tp.superType)
       case _ => false
     }
+    def refinementIsInvariant2(tp: Type): Boolean = tp match
+      case tp: SingletonType => true
+      case tp: ClassInfo     => !tp.cls.isOneOf(AbstractOrTrait) || tp.cls.isOneOf(Private | Sealed)
+      case tp: TypeProxy     => refinementIsInvariant2(tp.superType)
+      case _                 => false
 
-    def widenVariantParams(tp: Type) = tp match {
-      case tp @ AppliedType(tycon, args) =>
-        val args1 = args.zipWithConserve(tycon.typeParams)((arg, tparam) =>
-          if (tparam.paramVarianceSign != 0) TypeBounds.empty else arg
-        )
-        tp.derivedAppliedType(tycon, args1)
-      case tp =>
-        tp
-    }
+    extension (tp: Type) def isAbstract: Boolean = tp.stripped match
+      case _: TypeParamRef => true
+      case tp: TypeRef     => !tp.symbol.isClass
+      case _               => false
 
     val patternCls = patternTp.classSymbol
     val scrutineeCls = scrutineeTp.classSymbol
@@ -269,10 +271,11 @@ trait PatternTypeConstrainer { self: TypeComparer =>
           val result =
             tyconS.typeParams.lazyZip(argsS).lazyZip(argsP).forall { (param, argS, argP) =>
               val variance = param.paramVarianceSign
-              if variance == 0 || assumeInvariantRefinement ||
+              if variance == 0 || assumeInvariantRefinement
+                || refinementIsInvariant2(patternTp) && (argP.isAbstract || patternTp.argInfos.contains(argP))
                 // As a special case, when pattern and scrutinee types have the same type constructor,
                 // we infer better bounds for pattern-bound abstract types.
-                argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
+                || argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
               then
                 val TypeBounds(loS, hiS) = argS.bounds
                 val TypeBounds(loP, hiP) = argP.bounds

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

@@ -2074,9 +2074,9 @@ object SymDenotations {
         required: FlagSet = EmptyFlags, excluded: FlagSet = EmptyFlags)(using Context): Denotation =
       membersNamedNoShadowingBasedOnFlags(name, required, excluded).asSeenFrom(pre).toDenot(pre)
 
-    /** Compute tp.baseType(this) */
-    final def baseTypeOf(tp: Type)(using Context): Type = {
-      val btrCache = baseTypeCache
+    /** Compute tp.baseType(this) or tp.baseType(this, without) */
+    final def baseTypeOf(tp: Type, without: Option[Symbol] = None)(using Context): Type = {
+      val btrCache = if without.isEmpty then baseTypeCache else new BaseTypeMap()
       def inCache(tp: Type) = tp match
         case tp: CachedType => btrCache.contains(tp)
         case _ => false
@@ -2130,6 +2130,8 @@ object SymDenotations {
                   val baseTp =
                     if (tpSym eq symbol)
                       tp
+                    else if without.exists(tpSym eq _) then
+                      defn.AnyType
                     else if (isOwnThis)
                       if (clsd.baseClassSet.contains(symbol))
                         if (symbol.isStatic && symbol.typeParams.isEmpty) symbol.typeRef
@@ -2156,6 +2158,7 @@ object SymDenotations {
               btrCache(tp) = NoPrefix
               val baseTp =
                 if (tycon.typeSymbol eq symbol) tp
+                else if without.exists(tycon.typeSymbol eq _) then defn.AnyType
                 else (tycon.typeParams: @unchecked) match {
                   case LambdaParam(_, _) :: _ =>
                     recur(tp.superType)

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

@@ -1129,6 +1129,12 @@ object Types {
       }
     }
 
+    /** `basetype`, but ignoring any base classes that have the given `without` class symbol. */
+    final def baseTypeWithout(base: Symbol, without: Symbol)(using Context): Type =
+      base.denot match
+        case classd: ClassDenotation => classd.baseTypeOf(this, Some(without))
+        case _ => NoType
+
     def & (that: Type)(using Context): Type = {
       record("&")
       TypeComparer.glb(this, that)

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -770,16 +770,18 @@ object RefChecks {
           }
       }
 
-      /** Check that inheriting a case class does not constitute a variant refinement
-       *  of a base type of the case class. It is because of this restriction that we
-       *  can assume invariant refinement for case classes in `constrainPatternType`.
+      /** Check that inheriting a class does not constitute a variant refinement
+       *  of a base type of the class. It is because of this restriction that we
+       *  can assume invariant refinement for concrete classes in `constrainPatternType`.
        */
-      def checkCaseClassInheritanceInvariant() =
-        for (caseCls <- clazz.info.baseClasses.tail.find(_.is(Case)))
-          for (baseCls <- caseCls.info.baseClasses.tail)
+      def checkClassInheritanceInvariant() =
+        for (middle <- clazz.info.baseClasses.tail.filter(!_.isTransparentTrait))
+          for (baseCls <- middle.info.baseClasses.tail)
             if (baseCls.typeParams.exists(_.paramVarianceSign != 0))
-              for (problem <- variantInheritanceProblems(baseCls, caseCls, "non-variant", "case "))
+              val middleStr = if middle.is(Case) then "case " else ""
+              for (problem <- variantInheritanceProblems(baseCls, middle, "variant", middleStr))
                 report.errorOrMigrationWarning(problem(), clazz.srcPos, from = `3.0`)
+
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
@@ -788,7 +790,7 @@ object RefChecks {
         report.error(abstractErrorMessage, clazz.srcPos)
 
       checkMemberTypesOK()
-      checkCaseClassInheritanceInvariant()
+      checkClassInheritanceInvariant()
     }
 
     if (!clazz.is(Trait)) {
@@ -825,16 +827,28 @@ object RefChecks {
      */
     def variantInheritanceProblems(
         baseCls: Symbol, middle: Symbol, baseStr: String, middleStr: String): Option[() => String] = {
+      if baseCls == middle then return None
       val superBT = self.baseType(middle)
-      val thisBT = self.baseType(baseCls)
       val combinedBT = superBT.baseType(baseCls)
-      if (combinedBT =:= thisBT) None // ok
+      val withoutMiddleBT = self.baseTypeWithout(baseCls, middle)
+      val allOk = (combinedBT, withoutMiddleBT) match
+        case (AppliedType(tycon, args1), AppliedType(_, args2)) =>
+          val superBTArgs = superBT.argInfos.toSet
+          tycon.typeParams.lazyZip(args1).lazyZip(args2).forall { (param, arg1, arg2) =>
+            if superBTArgs.contains(arg1) then
+              val variance = param.paramVarianceSign
+              (variance > 0 || (arg2 <:< arg1)) &&
+              (variance < 0 || (arg1 <:< arg2))
+            else true // e.g. CovBoth in neg/i11834
+          }
+        case _ => combinedBT =:= self.baseType(baseCls)
+      if allOk then None // ok
       else
         Some(() =>
           em"""illegal inheritance: $clazz inherits conflicting instances of $baseStr base $baseCls.
               |
-              |  Direct basetype: $thisBT
-              |  Basetype via $middleStr$middle: $combinedBT""")
+              |  Basetype via $middleStr$middle: $combinedBT
+              |  Basetype without $middleStr$middle: $withoutMiddleBT""")
     }
 
     /* Returns whether there is a symbol declared in class `inclazz`

tests/neg-custom-args/isInstanceOf/3324g.erasure.scala

@@ -0,0 +1,12 @@
+// like neg-custom-args/isInstanceOf/3324g,
+// but verifying the fatal type test/unchecked warning
+// emitted during Erasure
+// by not being trumped by the fatal refcheck warning on C subclass
+class Test {
+  trait A[+T]
+  class B[T] extends A[T]
+
+  def quux[T](a: A[T]): Unit = a match {
+    case _: B[T] => // error
+  }
+}

tests/neg-custom-args/isInstanceOf/3324g.scala

@@ -1,19 +1,5 @@
 class Test {
   trait A[+T]
   class B[T] extends A[T]
-  class C[T] extends B[Any] with A[T]
-
-  def foo[T](c: C[T]): Unit = c match {
-    case _: B[T] => // error
-  }
-
-  def bar[T](b: B[T]): Unit = b match {
-    case _: A[T] =>
-  }
-
-  def quux[T](a: A[T]): Unit = a match {
-    case _: B[T] => // error!!
-  }
-
-  quux(new C[Int])
+  class C[T] extends B[Any] with A[T] // error
 }

tests/neg/i11018.scala

@@ -13,9 +13,19 @@ trait CTrait[+A](val a: A) {
 trait DTrait[+B] extends CTrait[B]
 trait DClass[+B] extends CClass[B]
 
-final class F1 extends DTrait[Foo] with CTrait[Bar](new Bar) // error: illegal parameter
-final class F2 extends CTrait[Bar](new Bar) with DTrait[Foo] // error: illegal parameter
-final class F3 extends DClass[Foo] with CClass[Bar](new Bar) // error: illegal parameter
-final class F4 extends CClass[Bar](new Bar) with DClass[Foo] // error: illegal parameter
+final class F1 // error: illegal inheritance
+  extends DTrait[Foo]
+  with CTrait[Bar](new Bar) // error: illegal parameter
+final class F2 // error: illegal inheritance
+  extends CTrait[Bar](new Bar) // error: illegal parameter
+  with DTrait[Foo]
+final class F3 // error: illegal inheritance
+  extends DClass[Foo]
+  with CClass[Bar](new Bar) // error: illegal parameter
+final class F4 // error: illegal inheritance
+  extends CClass[Bar](new Bar) // error: illegal parameter
+  with DClass[Foo]
 
-final class F5 extends DTrait[Foo] with CTrait[Foo & Bar](new Bar with Foo { def name = "hello"}) // ok
+final class F5 // error: illegal inheritance
+  extends DTrait[Foo]
+  with CTrait[Foo & Bar](new Bar with Foo { def name = "hello"})