add error tests

Author: noti0na1

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

@@ -161,7 +161,6 @@ object ErrorReporting {
           case fail: Implicits.NoMatchingImplicits => // do nothing
           case _ => attempts += ((failure.tree, ""))
       if foundWithoutNull then
-        // !!! TODO: The need a test that tests this error message with a check file
         i""".
           |Since explicit-nulls is enabled, the selection is rejected because
           |${qualType.widen} could be null at runtime.

tests/explicit-nulls/neg-separate/upper-bounds/TestA_1.scala

@@ -1,22 +1,30 @@
-val x1: String = ???
-val x2: String | Null = ???
+// Test Null type in bounds
 
-// T has to be nullable type
-def f1[T >: Null <: AnyRef | Null](x: T): T = x
+class Test {
+  val x1: String = ???
+  val x2: String | Null = ???
 
-// Null is no longer a subtype of AnyRef, impossible to apply this method directly.
-// We can bypass this restriction by importing unsafeNulls.
-def f2[T >: Null <: AnyRef](x: T): T = x
+  // T has to be nullable type
+  def f1[T >: Null <: AnyRef | Null](x: T): T = x
 
-def nullOf[T >: Null <: AnyRef | Null]: T = null
+  // Null is no longer a subtype of AnyRef, so it is impossible to apply this method directly.
+  // However, defining this kind of functions is allowed.
+  // We can bypass this restriction by importing unsafeNulls.
+  def f2[T >: Null <: AnyRef](x: T): T = x
 
-def g = {
-  f1(x1)
-  f1(x2)
+  def nullOf[T >: Null <: AnyRef | Null]: T = null
 
-  f2(x1) // error
-  f2(x2) // error
+  def g = {
+    f1(x1)
+    f1(x2)
 
-  val n1: String = nullOf // error
-  val n3: String | Null = nullOf
+    f2(x1) // error
+    f2(x2) // error
+
+    val n1: String = nullOf // error
+    val n3: String | Null = nullOf
+  }
+
+  // Bounds in class definition is strictly checked
+  class A[T >: Null <: AnyRef] {} // error: conflicting bounds Null <: ... <: AnyRef
 }

tests/explicit-nulls/neg-separate/upper-bounds/TestB_2.scala

@@ -0,0 +1,16 @@
+-- [E008] Not Found Error: tests/explicit-nulls/neg/nn-basic.scala:3:21 ------------------------------------------------
+3 |  val s2: String = s.trim // error
+  |                   ^^^^^^
+  |                   value trim is not a member of String | Null.
+  |                   Since explicit-nulls is enabled, the selection is rejected because
+  |                   String | Null could be null at runtime.
+  |                   If you want to select trim without checking for a null value,
+  |                   insert a .nn before .trim or import scala.language.unsafeNulls.
+-- [E008] Not Found Error: tests/explicit-nulls/neg/nn-basic.scala:5:17 ------------------------------------------------
+5 |  val l: Int = s.length // error
+  |               ^^^^^^^^
+  |               value length is not a member of String | Null.
+  |               Since explicit-nulls is enabled, the selection is rejected because
+  |               String | Null could be null at runtime.
+  |               If you want to select length without checking for a null value,
+  |               insert a .nn before .length or import scala.language.unsafeNulls.

tests/explicit-nulls/neg/bounds.scala

@@ -0,0 +1,6 @@
+class Test {
+  val s: String | Null = ???
+  val s2: String = s.trim // error
+  val s3: String | Null = s.nn.trim
+  val l: Int = s.length // error
+}