Only final lazy vals can be paths.

Reason: They might be overridden by other lazy vals
that are not realizable, and therefore risk creating
bad bounds.

Author: odersky

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

@@ -525,7 +525,9 @@ object SymDenotations {
     /** Is this a denotation of a realizable term (or an arbitrary type)? */
     final def isRealizable(implicit ctx: Context) =
       is(Stable) || isType || {
-        val isRealizable = !is(Lazy, butNot = Module) || ctx.realizability(info) == TypeOps.Realizable
+        val isRealizable =
+          !is(Lazy, butNot = Module) ||
+          is(Final) && ctx.realizability(info) == TypeOps.Realizable
         isRealizable && { setFlag(Stable); true }
       }
 

test/dotc/tests.scala

@@ -156,7 +156,8 @@ class tests extends CompilerTest {
   @Test def neg_i705 = compileFile(negDir, "i705-inner-value-class", xerrors = 7)
   @Test def neg_i866 = compileFile(negDir, "i866", xerrors = 2)
   @Test def neg_i974 = compileFile(negDir, "i974", xerrors = 2)
-  @Test def neg_i1050 = compileFile(negDir, "i1050", xerrors = 3)
+  @Test def neg_i1050 = compileFile(negDir, "i1050", xerrors = 5)
+  @Test def neg_i1050a = compileFile(negDir, "i1050a", xerrors = 2)
   @Test def neg_moduleSubtyping = compileFile(negDir, "moduleSubtyping", xerrors = 4)
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)
   @Test def neg_instantiateAbstract = compileFile(negDir, "instantiateAbstract", xerrors = 8)

tests/neg/i1050.scala

@@ -39,7 +39,7 @@ object Tiark1 {
     trait A { type L <: Nothing }
     trait B { type L >: Any}
     trait U {
-      val p: B
+      lazy val p: B
       def brand(x: Any): p.L = x // error: not final
     }
     trait V extends U {
@@ -53,7 +53,7 @@ object Tiark2 {
     trait B { type L >: Any}
     trait U {
       type X <: B
-      val p: X
+      lazy val p: X
       def brand(x: Any): p.L = x // error: not final
     }
     trait V extends U {

tests/neg/i1050a.scala

@@ -0,0 +1,29 @@
+object Tiark1 {
+    trait A { type L <: Nothing }
+    trait B { type L >: Any}
+    trait U {
+      val p: B
+      def brand(x: Any): p.L = x // error: not final
+    }
+    trait V extends U {
+      lazy val p: A & B = ???
+    }
+    val v = new V {}
+    v.brand("boom!")
+}
+object Tiark2 {
+    trait A { type L <: Nothing }
+    trait B { type L >: Any}
+    trait U {
+      type X <: B
+      val p: X
+      def brand(x: Any): p.L = x // error: not final
+    }
+    trait V extends U {
+      type X = B & A
+      lazy val p: X = ???
+    }
+    val v = new V {}
+    v.brand("boom!"): Nothing
+}
+