Remove special overriding logic for explicit nulls (#22243)

Fixes #22071

Author: noti0na1

Diff for: compiler/src/dotty/tools/dotc/core/Contexts.scala

@@ -786,9 +786,6 @@ object Contexts {
     def withNotNullInfos(infos: List[NotNullInfo]): Context =
       if !c.explicitNulls || (c.notNullInfos eq infos) then c else c.fresh.setNotNullInfos(infos)
 
-    def relaxedOverrideContext: Context =
-      c.withModeBits(c.mode &~ Mode.SafeNulls | Mode.RelaxedOverriding)
-
   // TODO: Fix issue when converting ModeChanges and FreshModeChanges to extension givens
   extension (c: Context) {
     final def withModeBits(mode: Mode): Context =

Diff for: compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -478,12 +478,11 @@ object Denotations {
                 else if sym1.is(Method) && !sym2.is(Method) then 1
                 else 0
 
-          val relaxedOverriding = ctx.explicitNulls && (sym1.is(JavaDefined) || sym2.is(JavaDefined))
           val matchLoosely = sym1.matchNullaryLoosely || sym2.matchNullaryLoosely
 
-          if symScore <= 0 && info2.overrides(info1, relaxedOverriding, matchLoosely, checkClassInfo = false) then
+          if symScore <= 0 && info2.overrides(info1, matchLoosely, checkClassInfo = false) then
             denot2
-          else if symScore >= 0 && info1.overrides(info2, relaxedOverriding, matchLoosely, checkClassInfo = false) then
+          else if symScore >= 0 && info1.overrides(info2, matchLoosely, checkClassInfo = false) then
             denot1
           else
             val jointInfo = infoMeet(info1, info2, safeIntersection)

Diff for: compiler/src/dotty/tools/dotc/core/JavaNullInterop.scala

@@ -6,6 +6,8 @@ import Flags.JavaDefined
 import StdNames.nme
 import Symbols.*
 import Types.*
+import dotty.tools.dotc.reporting.*
+import dotty.tools.dotc.core.Decorators.i
 
 /** This module defines methods to interpret types of Java symbols, which are implicitly nullable in Java,
  *  as Scala types, which are explicitly nullable.
@@ -51,7 +53,7 @@ object JavaNullInterop {
    *
    *  But the selection can throw an NPE if the returned value is `null`.
    */
-  def nullifyMember(sym: Symbol, tp: Type, isEnumValueDef: Boolean)(using Context): Type = {
+  def nullifyMember(sym: Symbol, tp: Type, isEnumValueDef: Boolean)(using Context): Type = trace(i"nullifyMember ${sym}, ${tp}"){
     assert(ctx.explicitNulls)
     assert(sym.is(JavaDefined), "can only nullify java-defined members")
 

Diff for: compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -105,7 +105,7 @@ object Mode {
 
   /** Use previous Scheme for implicit resolution. Currently significant
    *  in 3.0-migration where we use Scala-2's scheme instead and in 3.5 and 3.6-migration
-   *  where we use the previous scheme up to 3.4 for comparison with the new scheme. 
+   *  where we use the previous scheme up to 3.4 for comparison with the new scheme.
    */
   val OldImplicitResolution: Mode = newMode(15, "OldImplicitResolution")
 
@@ -163,11 +163,6 @@ object Mode {
    */
   val ForceInline: Mode = newMode(29, "ForceInline")
 
-  /** This mode is enabled when we check Java overriding in explicit nulls.
-   *  Type `Null` becomes a subtype of non-primitive value types in TypeComparer.
-   */
-  val RelaxedOverriding: Mode = newMode(30, "RelaxedOverriding")
-
   /** Skip inlining of methods. */
   val NoInline: Mode = newMode(31, "NoInline")
 }

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -967,17 +967,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             || compareGADT
             || tryLiftedToThis1
           case _ =>
-            // `Mode.RelaxedOverriding` is only enabled when checking Java overriding
-            // in explicit nulls, and `Null` becomes a bottom type, which allows
-            // `T | Null` being a subtype of `T`.
-            // A type variable `T` from Java is translated to `T >: Nothing <: Any`.
-            // However, `null` can always be a value of `T` for Java side.
-            // So the best solution here is to let `Null` be a subtype of non-primitive
-            // value types temporarily.
             def isNullable(tp: Type): Boolean = tp.dealias match
               case tp: TypeRef =>
                 val tpSym = tp.symbol
-                ctx.mode.is(Mode.RelaxedOverriding) && !tpSym.isPrimitiveValueClass ||
                 tpSym.isNullableClass
               case tp: TermRef =>
                 // https://scala-lang.org/files/archive/spec/2.13/03-types.html#singleton-types

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1160,17 +1160,14 @@ object Types extends TypeUtils {
      *
      *  @param isSubType      a function used for checking subtype relationships.
      */
-    final def overrides(that: Type, relaxedCheck: Boolean, matchLoosely: => Boolean, checkClassInfo: Boolean = true,
+    final def overrides(that: Type, matchLoosely: => Boolean, checkClassInfo: Boolean = true,
                         isSubType: (Type, Type) => Context ?=> Boolean = (tp1, tp2) => tp1 frozen_<:< tp2)(using Context): Boolean = {
-      val overrideCtx = if relaxedCheck then ctx.relaxedOverrideContext else ctx
-      inContext(overrideCtx) {
-        !checkClassInfo && this.isInstanceOf[ClassInfo]
-        || isSubType(this.widenExpr, that.widenExpr)
-        || matchLoosely && {
-            val this1 = this.widenNullaryMethod
-            val that1 = that.widenNullaryMethod
-            ((this1 `ne` this) || (that1 `ne` that)) && this1.overrides(that1, relaxedCheck, false, checkClassInfo)
-          }
+      !checkClassInfo && this.isInstanceOf[ClassInfo]
+      || isSubType(this.widenExpr, that.widenExpr)
+      || matchLoosely && {
+        val this1 = this.widenNullaryMethod
+        val that1 = that.widenNullaryMethod
+        ((this1 `ne` this) || (that1 `ne` that)) && this1.overrides(that1, false, checkClassInfo)
       }
     }
 
@@ -1196,8 +1193,8 @@ object Types extends TypeUtils {
      */
     def matches(that: Type)(using Context): Boolean = {
       record("matches")
-      val overrideCtx = if ctx.explicitNulls then ctx.relaxedOverrideContext else ctx
-      TypeComparer.matchesType(this, that, relaxed = !ctx.phase.erasedTypes)(using overrideCtx)
+      withoutMode(Mode.SafeNulls)(
+        TypeComparer.matchesType(this, that, relaxed = !ctx.phase.erasedTypes))
     }
 
     /** This is the same as `matches` except that it also matches => T with T and

Diff for: compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala

@@ -225,11 +225,7 @@ object OverridingPairs:
             }
       )
     else
-      // releaxed override check for explicit nulls if one of the symbols is Java defined,
-      // force `Null` to be a subtype of non-primitive value types during override checking.
-      val relaxedOverriding = ctx.explicitNulls && (member.is(JavaDefined) || other.is(JavaDefined))
       member.name.is(DefaultGetterName) // default getters are not checked for compatibility
-      || memberTp.overrides(otherTp, relaxedOverriding,
-            member.matchNullaryLoosely || other.matchNullaryLoosely || fallBack, isSubType = isSubType)
+      || memberTp.overrides(otherTp, member.matchNullaryLoosely || other.matchNullaryLoosely || fallBack, isSubType = isSubType)
 
 end OverridingPairs

Diff for: compiler/src/dotty/tools/dotc/transform/ResolveSuper.scala

@@ -111,11 +111,8 @@ object ResolveSuper {
         // of the superaccessor's type, see i5433.scala for an example where this matters
         val otherTp = other.asSeenFrom(base.thisType).info
         val accTp = acc.asSeenFrom(base.thisType).info
-        // Since the super class can be Java defined,
-        // we use relaxed overriding check for explicit nulls if one of the symbols is Java defined.
-        // This forces `Null` to be a subtype of non-primitive value types during override checking.
-        val relaxedOverriding = ctx.explicitNulls && (sym.is(JavaDefined) || acc.is(JavaDefined))
-        if !otherTp.overrides(accTp, relaxedOverriding, matchLoosely = true) then
+
+        if !otherTp.overrides(accTp, matchLoosely = true) then
           report.error(IllegalSuperAccessor(base, memberName, targetName, acc, accTp, other.symbol, otherTp), base.srcPos)
       bcs = bcs.tail
     }

Diff for: compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -906,7 +906,7 @@ object RefChecks {
             for (mbrd <- self.member(name).alternatives) {
               val mbr = mbrd.symbol
               val mbrType = mbr.info.asSeenFrom(self, mbr.owner)
-              if (!mbrType.overrides(mbrd.info, relaxedCheck = false, matchLoosely = true))
+              if (!mbrType.overrides(mbrd.info, matchLoosely = true))
                 report.errorOrMigrationWarning(
                   em"""${mbr.showLocated} is not a legal implementation of `$name` in $clazz
                       |  its type             $mbrType

Diff for: project/Build.scala

@@ -1066,7 +1066,6 @@ object Build {
     // compiler is updated.
     // Then, the next step is to enable flexible types by default and reduce the use of
     // `unsafeNulls`.
-    scalacOptions ++= Seq("-Yno-flexible-types"),
     packageAll := {
       (`scala3-compiler` / packageAll).value ++ Seq(
         "scala3-compiler" -> (Compile / packageBin).value.getAbsolutePath,
@@ -1460,10 +1459,6 @@ object Build {
     .dependsOn(`scala3-compiler-bootstrapped`, `scala3-library-bootstrapped`, `scala3-presentation-compiler-testcases` % "test->test")
     .settings(presentationCompilerSettings)
     .settings(scala3PresentationCompilerBuildInfo)
-    .settings(
-      // Add `-Yno-flexible-types` flag for bootstrap, see comments for `bootstrappedDottyCompilerSettings`
-      Compile / scalacOptions +=  "-Yno-flexible-types"
-    )
 
   def scala3PresentationCompilerBuildInfo =
     Seq(