Fix derivesFrom false negative in provablyDisjointClasses

compiler/src/dotty/tools/backend/jvm/CodeGen.scala

@@ -85,8 +85,8 @@ class CodeGen(val int: DottyBackendInterface, val primitives: DottyPrimitives)(
         case ex: InterruptedException => throw ex
         case ex: CompilationUnit.SuspendException => throw ex
         case ex: Throwable =>
-          ex.printStackTrace()
-          report.error(s"Error while emitting ${unit.source}\n${ex.getMessage}", NoSourcePosition)
+          if !ex.isInstanceOf[TypeError] then ex.printStackTrace()
+          report.error(s"Error while emitting ${unit.source}\n${ex.getMessage}", cd.sourcePos)
 
 
     def genTastyAndSetAttributes(claszSymbol: Symbol, store: ClassNode): Unit =

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

@@ -865,9 +865,23 @@ object SymDenotations {
      *  and is the denoting symbol also different from `Null` or `Nothing`?
      *  @note  erroneous classes are assumed to derive from all other classes
      *         and all classes derive from them.
+     *  @note may return a false negative when `this.info.isInstanceOf[TempClassInfo]`.
      */
     def derivesFrom(base: Symbol)(using Context): Boolean = false
 
+    /** Could `this` derive from `base` now or in the future.
+     *  For concistency with derivesFrom, the info is only forced when this is a ClassDenotation.
+     *  If the info is a TempClassInfo then the baseClassSet may be temporarily approximated as empty.
+     *  This is problematic when stability of `!derivesFrom(base)` is assumed for soundness,
+     *  e.g., in `TypeComparer#provablyDisjointClasses`.
+     *  @note may return a false positive when `this.info.isInstanceOf[TempClassInfo]`.
+     */
+    final def mayDeriveFrom(base: Symbol)(using Context): Boolean =
+      this.isInstanceOf[ClassDenotation] && (info.isInstanceOf[TempClassInfo] || derivesFrom(base))
+
+    final def derivesFrom(base: Symbol, defaultIfUnknown: Boolean)(using Context): Boolean =
+      if defaultIfUnknown/*== true*/ then mayDeriveFrom(base) else derivesFrom(base)
+
     /** Is this a Scala or Java annotation ? */
     def isAnnotation(using Context): Boolean =
       isClass && (derivesFrom(defn.AnnotationClass) || is(JavaAnnotation))

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

@@ -3131,9 +3131,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
        * unique value derives from the class.
        */
       case (tp1: SingletonType, tp2) =>
-        !tp1.derivesFrom(tp2.classSymbol)
+        !tp1.derivesFrom(tp2.classSymbol, defaultIfUnknown = true)
       case (tp1, tp2: SingletonType) =>
-        !tp2.derivesFrom(tp1.classSymbol)
+        !tp2.derivesFrom(tp1.classSymbol, defaultIfUnknown = true)
 
       /* Now both sides are possibly-parameterized class types `p.C[Ts]` and `q.D[Us]`.
        *
@@ -3189,7 +3189,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             val cls2BaseClassSet = SymDenotations.BaseClassSet(cls2.classDenot.baseClasses)
             val commonBaseClasses = cls1.classDenot.baseClasses.filter(cls2BaseClassSet.contains(_))
             def isAncestorOfOtherBaseClass(cls: ClassSymbol): Boolean =
-              commonBaseClasses.exists(other => (other ne cls) && other.derivesFrom(cls))
+              commonBaseClasses.exists(other => (other ne cls) && other.mayDeriveFrom(cls))
             val result = commonBaseClasses.exists { baseClass =>
               !isAncestorOfOtherBaseClass(baseClass) && isBaseTypeWithDisjointArguments(baseClass, innerPending)
             }
@@ -3230,7 +3230,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       .filter(child => child.exists && child != cls)
 
     def eitherDerivesFromOther(cls1: Symbol, cls2: Symbol): Boolean =
-      cls1.derivesFrom(cls2) || cls2.derivesFrom(cls1)
+      cls1.mayDeriveFrom(cls2) || cls2.mayDeriveFrom(cls1)
 
     def smallestNonTraitBase(cls: Symbol): Symbol =
       val classes = if cls.isClass then cls.asClass.baseClasses else cls.info.classSymbols

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

@@ -270,15 +270,15 @@ object Types extends TypeUtils {
     /** True if this type is an instance of the given `cls` or an instance of
      *  a non-bottom subclass of `cls`.
      */
-    final def derivesFrom(cls: Symbol)(using Context): Boolean = {
+    final def derivesFrom(cls: Symbol, defaultIfUnknown: Boolean = false)(using Context): Boolean = {
       def isLowerBottomType(tp: Type) =
         tp.isBottomType
         && (tp.hasClassSymbol(defn.NothingClass)
             || cls != defn.NothingClass && !cls.isValueClass)
       def loop(tp: Type): Boolean = try tp match
         case tp: TypeRef =>
           val sym = tp.symbol
-          if (sym.isClass) sym.derivesFrom(cls) else loop(tp.superType)
+          if (sym.isClass) sym.derivesFrom(cls, defaultIfUnknown) else loop(tp.superType)
         case tp: AppliedType =>
           tp.superType.derivesFrom(cls)
         case tp: MatchType =>

tests/neg/i17132.min.check

@@ -0,0 +1,31 @@
+-- Error: tests/neg/i17132.min.scala:4:7 -------------------------------------------------------------------------------
+4 |class Q[T <: P[Any]] extends P[R[T]] // error
+  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |       Recursion limit exceeded.
+  |       Maybe there is an illegal cyclic reference?
+  |       If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
+  |       For the unprocessed stack trace, compile with -Xno-enrich-error-messages.
+  |       A recurring operation is (inner to outer):
+  |
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         ...
+  |
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  t match ...
+  |         reduce type  T match ...

tests/neg/i17132.min.scala

@@ -0,0 +1,9 @@
+
+class P[T]
+//class Q[T] extends P[R[T]] // ok
+class Q[T <: P[Any]] extends P[R[T]] // error
+//type Q[T <: P[Any]] <: P[R[T]] // ok
+
+type R[U] = U match
+  case Q[t] => R[t]
+  case P[t] => t

tests/neg/i17132.scala

@@ -0,0 +1,11 @@
+
+sealed trait Transformation[T]
+
+case object Count extends Transformation[Int]
+case class MultiTransformation[T1 <: Transformation[?], T2 <: Transformation[?]](t1: T1, t2: T2) // error cyclic
+  extends Transformation[MultiTransformationResult[T1, T2]]
+
+type MultiTransformationResult[T1 <: Transformation[?], T2 <: Transformation[?]] <: Tuple = (T1, T2) match {
+  case (Transformation[t], MultiTransformation[t1, t2]) => t *: MultiTransformationResult[t1, t2]
+  case (Transformation[t1], Transformation[t2]) => (t1, t2)
+}