Relax nullability checks on FlexibleTypes

Author: HarrisL2

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -942,7 +942,7 @@ object CaptureSet:
      *  is not derived from some other variable.
      */
     protected def ids(using Context): String =
-      def descr = getClass.getSimpleName.nn.take(1)
+      def descr = getClass.getSimpleName.take(1)
       val trail = this.match
         case dv: DerivedVar =>
           def summary = if ctx.settings.YccVerbose.value then dv.summarize else descr

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

@@ -47,7 +47,7 @@ object NullOpsDecorator:
 
     /** Is self (after widening and dealiasing) a type of the form `T | Null`? */
     def isNullableUnion(using Context): Boolean = {
-      val stripped = self.stripNull()
+      val stripped = self.stripNull(stripFlexibleTypes = false)
       stripped ne self
     }
   end extension

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

@@ -322,7 +322,7 @@ object SymbolLoaders {
           val fullName = pkg.name
           val name =
             if (packageName.isEmpty) fullName
-            else fullName.substring(packageName.length + 1).nn
+            else fullName.substring(packageName.length + 1)
 
           enterPackage(root.symbol, name.toTermName,
             (module, modcls) => new PackageLoader(module, classPath))

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

@@ -3376,12 +3376,12 @@ object TypeComparer {
   }
 
   private[core] def show(res: Any)(using Context): String =
-    if ctx.settings.YexplainLowlevel.value then String.valueOf(res).nn
+    if ctx.settings.YexplainLowlevel.value then String.valueOf(res)
     else res match
       case ClassInfo(_, cls, _, _, _) => cls.showLocated
       case bounds: TypeBounds => i"type bounds [$bounds]"
       case res: printing.Showable => res.show
-      case _ => String.valueOf(res).nn
+      case _ => String.valueOf(res)
 
   /** The approximation state indicates how the pair of types currently compared
    *  relates to the types compared originally.

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

@@ -382,7 +382,7 @@ object Types extends TypeUtils {
     /** Is this type guaranteed not to have `null` as a value? */
     final def isNotNull(using Context): Boolean = this match {
       case tp: ConstantType => tp.value.value != null
-      case tp: FlexibleType => false
+      case tp: FlexibleType => true
       case tp: ClassInfo => !tp.cls.isNullableClass && !tp.isNothingType
       case tp: AppliedType => tp.superType.isNotNull
       case tp: TypeBounds => tp.hi.isNotNull

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

@@ -1210,7 +1210,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     }
     catch {
       case ex: FromDigitsException =>
-        report.error(ex.getMessage.nn, tree.srcPos)
+        report.error(ex.getMessage, tree.srcPos)
         tree.kind match {
           case Whole(_) => lit(0)
           case _ => lit(0.0)

tests/explicit-nulls/warn/unnecessary-nn.scala

@@ -10,7 +10,7 @@ def f5a[T <: String](s: T): String = s.nn // warn
 
 // flexible types
 def f7(s: String|Null): String = "".concat(s.nn) // warn
-def f8(s: String): String = s.trim().nn // OK because the .nn could be useful as a dynamic null check
+def f8(s: String): String = s.trim().nn // warn
 
 
 def f9(s: String|Null): String =