Reject all explicitly written type references with bad bounds

... except for type parameters, since these will need to be instantiated
with good types themselves. Fixes further unsoundness examples added to
neg/i15569.scala.

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

@@ -280,7 +280,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
                ctx
           super.transform(tree)(using gadtCtx)
         case tree: Ident =>
-          if tree.isType then
+          if tree.isType && !tree.symbol.is(Param) then
+            Checking.checkGoodBounds(tree.tpe, tree.srcPos)
             checkNotPackage(tree)
           else
             if tree.symbol.is(Inline) && !Inlines.inInlineMethod then
@@ -295,6 +296,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             ctx.compilationUnit.needsInlining = true
           if name.isTypeName then
             Checking.checkRealizable(qual.tpe, qual.srcPos)
+            Checking.checkGoodBounds(tree.tpe, tree.srcPos)
             withMode(Mode.Type)(super.transform(checkNotPackage(tree)))
           else
             checkNoConstructorProxy(tree)
@@ -345,12 +347,6 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           val tree1 @ TypeApply(fn, args) = normalizeTypeArgs(tree)
           for arg <- args do
             checkInferredWellFormed(arg)
-            val isInferred = arg.isInstanceOf[InferredTypeTree] || arg.span.isSynthetic
-            if !isInferred then
-              // only check explicit type arguments. We rely on inferred type arguments
-              // to either have good bounds (if they come from a constraint), or be derived
-              // from values that recursively need to have good bounds.
-              Checking.checkGoodBounds(arg.tpe, arg.srcPos)
           if (fn.symbol != defn.ChildAnnot.primaryConstructor)
             // Make an exception for ChildAnnot, which should really have AnyKind bounds
             Checking.checkBounds(args, fn.tpe.widen.asInstanceOf[PolyType])

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

@@ -87,12 +87,12 @@ object Checking {
     checkBounds(args, tl.paramInfos, _.substParams(tl, _))
 
   def checkGoodBounds(tpe: Type, pos: SrcPos)(using Context): Boolean =
-    def recur(tp: Type) = tp.dealias match
+    def recur(tp: Type): Boolean = tp.dealias match
       case tp: TypeRef =>
-        checkGoodBounds(tp.info, pos)
-      case TypeBounds(lo, hi) if !(lo <:< hi) =>
-        val argStr = if tp eq tpe then "" else i" $tpe"
-        report.error(i"type argument$argStr has potentially unrealizable bounds $tp", pos)
+        recur(tp.info)
+      case tp @ TypeBounds(lo, hi) if !(lo <:< hi) =>
+        val tpStr = if tp eq tpe then "" else i" $tpe"
+        report.error(i"type$tpStr has potentially conflicting bounds $tp", pos)
         false
       case _ =>
         true

tests/neg-strict/i1050.scala

@@ -100,7 +100,7 @@ object Indirect {
   trait U {
     trait X {
       val q: A & B = ???
-      type M = q.L
+      type M = q.L // error: conflicting bounds
     }
     final lazy val p: X = ???
     def brand(x: Any): p.M = x // error: conflicting bounds
@@ -119,7 +119,7 @@ object Indirect2 {
     }
     trait X {
       val q: Y = ???
-      type M = q.r.L
+      type M = q.r.L  // error: conflicting bounds
     }
     final lazy val p: X = ???
     def brand(x: Any): p.M = x // error: conflicting bounds
@@ -152,7 +152,7 @@ object Rec2 {
     }
     trait X {
       val q: Y = ???
-      type M = q.r.L
+      type M = q.r.L // error: conflicting bounds
     }
     final lazy val p: X = ???
     def brand(x: Any): p.M = x // error: conflicting bounds
@@ -171,7 +171,7 @@ object Indirect3 {
     }
     trait X {
       val q: Y = ???
-      type M = q.r.L
+      type M = q.r.L // error: conflicting bounds
     }
     final lazy val p: X = ???
     def brand(x: Any): p.M = x // error: conflicting bounds

tests/init/neg/i11572.scala → tests/neg/i11572.scala

@@ -5,13 +5,13 @@ class A {
   trait Bounded {
     type T >: Cov[Int] <: Cov[String]
   }
-  val t: Bounded = new Bounded {     // error
+  val t: Bounded = new Bounded {
     // Note: using this instead of t produces an error (as expected)
-    override type T >: t.T <: t.T
+    override type T >: t.T <: t.T  // error // error (conflicting bounds)
   }
 
   val covInt = new Cov[Int] {
     override def get: Int = 3
   }
-  val str: String = ((covInt: t.T): Cov[String]).get // ClassCastException: class Integer cannot be cast to class String
+  val str: String = ((covInt: t.T): Cov[String]).get // error, was ClassCastException: class Integer cannot be cast to class String
 }

tests/neg/i13820.scala

@@ -0,0 +1,5 @@
+trait Expr { type T }
+
+def foo[A](e: Expr { type T = A }) = e match
+  case e1: Expr { type T <: Int } =>
+    val i: Int = ??? : e1.T // error: unrealizable bounds

tests/neg/i15568.check

@@ -1,4 +1,4 @@
 -- Error: tests/neg/i15568.scala:3:15 ----------------------------------------------------------------------------------
 3 |type Bar = Foo[? >: Int <: String] // error
   |               ^
-  |               type argument has potentially unrealizable bounds  >: Int <: String
+  |               type has potentially conflicting bounds  >: Int <: String

tests/neg/i15569.scala

@@ -13,3 +13,18 @@ def andThenSub[A, B, C](f: A <:< B, g: B <:< C): A <:< C =
   val unsound: Any <:< Nothing = andThenSub[Any, t, Nothing](summon, summon) // error
   unsound("hi :)")
 
+@main def Test3 = (None: Option[Foo[?]]) match {
+  case _: Option[Foo[t]] =>
+    val unsound: Nothing = (5 : Any) : t  // error
+    (unsound : Unit => Unit).apply(())
+}
+
+@main def Test4 =
+  type t >: Any <: Nothing
+  val unsound: Nothing = (5 : Any) : t  // error
+  (unsound : Unit => Unit).apply(())
+
+@main def Test5 =
+  type t >: Any <: Nothing
+  val unsound: List[Nothing] = List(5 : Any) : List[t]  // error
+  (unsound.head : Unit => Unit).apply(())

tests/neg/i5854.scala

@@ -0,0 +1,5 @@
+class B {
+  val a: String = (((1: Any): b.A): Nothing): String  // error
+  val b: { type A >: Any <: Nothing } = loop()
+  def loop(): Nothing = loop()
+}

tests/pos/i13820.scala

@@ -1,5 +1,5 @@
 trait Expr { type T }
 
-def foo[A](e: Expr { type T = A }) = e match
+def foo[A <: Int](e: Expr { type T = A }) = e match
   case e1: Expr { type T <: Int } =>
-    val i: Int = ??? : e1.T
+    val i: Int = ??? : e1.T