Fix #5494: Spurious unchecked warning when type testing an alias

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

@@ -503,7 +503,7 @@ object Trees {
     type ThisTree[-T >: Untyped] = If[T]
     def isInline = false
   }
-  class InlineIf[T >: Untyped] private[ast] (cond: Tree[T], thenp: Tree[T], elsep: Tree[T])(implicit @constructorOnly src: SourceFile)
+  class InlineIf[-T >: Untyped] private[ast] (cond: Tree[T], thenp: Tree[T], elsep: Tree[T])(implicit @constructorOnly src: SourceFile)
     extends If(cond, thenp, elsep) {
     override def isInline = true
     override def toString = s"InlineIf($cond, $thenp, $elsep)"
@@ -529,7 +529,7 @@ object Trees {
     type ThisTree[-T >: Untyped] = Match[T]
     def isInline = false
   }
-  class InlineMatch[T >: Untyped] private[ast] (selector: Tree[T], cases: List[CaseDef[T]])(implicit @constructorOnly src: SourceFile)
+  class InlineMatch[-T >: Untyped] private[ast] (selector: Tree[T], cases: List[CaseDef[T]])(implicit @constructorOnly src: SourceFile)
     extends Match(selector, cases) {
     override def isInline = true
     override def toString = s"InlineMatch($selector, $cases)"
@@ -579,7 +579,7 @@ object Trees {
   }
 
   /** Array(elems) */
-  class JavaSeqLiteral[T >: Untyped] private[ast] (elems: List[Tree[T]], elemtpt: Tree[T])(implicit @constructorOnly src: SourceFile)
+  class JavaSeqLiteral[-T >: Untyped] private[ast] (elems: List[Tree[T]], elemtpt: Tree[T])(implicit @constructorOnly src: SourceFile)
     extends SeqLiteral(elems, elemtpt) {
     override def toString: String = s"JavaSeqLiteral($elems, $elemtpt)"
   }

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

@@ -105,10 +105,9 @@ object TypeTestsCasts {
       debug.println("P1 : " + P1.show)
       debug.println("X : " + X.show)
 
-      P1 <:< X       // constraint P1
+      P1 <:< X
 
-      // use fromScala2x to avoid generating pattern bound symbols
-      maximizeType(P1, span, fromScala2x = true)
+      maximizeType(P1, span, fromScala2x = false)
 
       val res = P1 <:< P
       debug.println("P1 : " + P1.show)
@@ -117,7 +116,7 @@ object TypeTestsCasts {
       res
     }
 
-    def recur(X: Type, P: Type): Boolean = (X <:< P) || (P match {
+    def recur(X: Type, P: Type): Boolean = (X <:< P) || (P.dealias match {
       case _: SingletonType     => true
       case _: TypeProxy
       if isAbstract(P)          => false
@@ -138,8 +137,8 @@ object TypeTestsCasts {
           case _ =>
             // first try withou striping type parameters for performance
             X.classSymbol.exists && P.classSymbol.exists && !X.classSymbol.asClass.mayHaveCommonChild(P.classSymbol.asClass) ||
-            isClassDetermined(X, tpe)(ctx.fresh.setNewTyperState()) ||
-            isClassDetermined(stripTypeParam(X), tpe)(ctx.fresh.setNewTyperState())
+            isClassDetermined(X, tpe)(ctx.fresh.setNewTyperState().setFreshGADTBounds) ||
+            isClassDetermined(stripTypeParam(X), tpe)(ctx.fresh.setNewTyperState().setFreshGADTBounds)
         }
       case AndType(tp1, tp2)    => recur(X, tp1) && recur(X, tp2)
       case OrType(tp1, tp2)     => recur(X, tp1) && recur(X, tp2)

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

@@ -12,7 +12,7 @@ class Test {
   }
 
   def quux[T](a: A[T]): Unit = a match {
-    case _: B[T] => // should be an error!!
+    case _: B[T] => // error!!
   }
 
   quux(new C[Int])

tests/neg-custom-args/isInstanceOf/gadt.scala

@@ -0,0 +1,13 @@
+class Test {
+  trait A[+T]
+  class B[T] extends A[T]
+
+  class C
+  class D extends C
+
+  def quux(a: A[C]): Unit = a match {
+    case _: B[C] => // error!!
+  }
+
+  quux(new B[D])
+}

tests/neg-custom-args/isInstanceOf/i5495.scala

@@ -0,0 +1,12 @@
+class A
+class B
+
+type AorB = A | B
+
+def foo(any: Any) = any match {
+  case aorb: AorB =>
+  case _ =>
+}
+
+def bar[T](x: List[T]) = x.isInstanceof[List[Int]] // error
+

tests/neg-custom-args/isInstanceOf/or-type-trees.scala

@@ -0,0 +1,13 @@
+trait Tree
+trait Context
+
+def foo(myTree: Tree | (Context => Tree)) =
+  println(myTree.isInstanceOf[Tree])
+  myTree match
+    case treeFn: (Context => Tree) =>  // error
+    case _ =>
+
+def bar(myTree: Tree | (Context => Tree)) =
+  myTree match
+    case treeFn: (_ => _) =>       // ok
+    case _ =>

tests/neg-custom-args/isInstanceOf/patmat-applied.scala

@@ -0,0 +1,11 @@
+class A[-T]
+class B[T] extends A[T]
+
+object Test {
+  def foo(x: A[Null]) = x match {
+    case x: B[Null] =>
+    case _ =>
+  }
+}
+
+def bar[T](x: List[T]) = x.isInstanceof[List[Int]] // error