Add ctx.explicitNulls to guard some calls to stripNull and OrNull.unapply

Author: noti0na1

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

@@ -1451,7 +1451,7 @@ object Types extends TypeUtils {
      *  then the top-level union isn't widened. This is needed so that type inference can infer nullable types.
      */
     def widenUnion(using Context): Type = widen match
-      case tp: OrType =>
+      case tp: OrType if ctx.explicitNulls =>
         val tp1 = tp.stripNull(stripFlexibleTypes = false)
         if tp1 ne tp then
           val tp1Widen = tp1.widenUnionWithoutNull

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

@@ -176,7 +176,7 @@ trait TypeAssigner {
     val qualType = qual.tpe.widenIfUnstable
     def kind = if tree.isType then "type" else "value"
     val foundWithoutNull = qualType match
-      case OrNull(qualType1) if qualType1 <:< defn.ObjectType =>
+      case OrNull(qualType1) if ctx.explicitNulls && qualType1 <:< defn.ObjectType =>
         val name = tree.name
         val pre = maybeSkolemizePrefix(qualType1, name)
         reallyExists(qualType1.findMember(name, pre))

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

@@ -588,11 +588,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
    */
   def toNotNullTermRef(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match
     case ref: TermRef
-    if pt != LhsProto && // Ensure it is not the lhs of Assign
-    ctx.notNullInfos.impliesNotNull(ref) &&
-    // If a reference is in the context, it is already trackable at the point we add it.
-    // Hence, we don't use isTracked in the next line, because checking use out of order is enough.
-    !ref.usedOutOfOrder =>
+    if ctx.explicitNulls
+      && pt != LhsProto // Ensure it is not the lhs of Assign
+      && ctx.notNullInfos.impliesNotNull(ref)
+      // If a reference is in the context, it is already trackable at the point we add it.
+      // Hence, we don't use isTracked in the next line, because checking use out of order is enough.
+      && !ref.usedOutOfOrder =>
       ref match
         case OrNull(tpnn) => tree.cast(AndType(ref, tpnn))
         case _            => tree
@@ -2228,7 +2229,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         given Context = caseCtx
         val case1 = typedCase(cas, sel, wideSelType, tpe)
         caseCtx = Nullables.afterPatternContext(sel, case1.pat)
-        if !alreadyStripped && Nullables.matchesNull(case1) then
+        if ctx.explicitNulls && !alreadyStripped && Nullables.matchesNull(case1) then
           wideSelType = wideSelType.stripNull()
           alreadyStripped = true
         case1
@@ -2261,7 +2262,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       given Context = caseCtx
       val case1 = typedCase(cas, sel, wideSelType, pt)
       caseCtx = Nullables.afterPatternContext(sel, case1.pat)
-      if !alreadyStripped && Nullables.matchesNull(case1) then
+      if ctx.explicitNulls && !alreadyStripped && Nullables.matchesNull(case1) then
         wideSelType = wideSelType.stripNull()
         alreadyStripped = true
       case1