Only nullify tasty if Yflexify-tasty is set; Refine FlexibleType nullification rules

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -455,6 +455,7 @@ private sealed trait YSettings:
   val YnoKindPolymorphism: Setting[Boolean] = BooleanSetting(ForkSetting, "Yno-kind-polymorphism", "Disable kind polymorphism. (This flag has no effect)", deprecation = Deprecation.removed())
   val YexplicitNulls: Setting[Boolean] = BooleanSetting(ForkSetting, "Yexplicit-nulls", "Make reference types non-nullable. Nullable types can be expressed with unions: e.g. String|Null.")
   val YnoFlexibleTypes: Setting[Boolean] = BooleanSetting(ForkSetting, "Yno-flexible-types", "Disable turning nullable Java return types and parameter types into flexible types, which behave like abstract types with a nullable lower bound and non-nullable upper bound.")
+  val YnullifyTasty: Setting[Boolean] = BooleanSetting(ForkSetting, "Ynullify-tasty", "Apply nullification to Scala code compiled without -Yexplicit-nulls, when reading from tasty.")
   val YsafeInitGlobal: Setting[Boolean] = BooleanSetting(ForkSetting, "Ysafe-init-global", "Check safe initialization of global objects.")
   val YrequireTargetName: Setting[Boolean] = BooleanSetting(ForkSetting, "Yrequire-targetName", "Warn if an operator is defined without a @targetName annotation.")
   val YrecheckTest: Setting[Boolean] = BooleanSetting(ForkSetting, "Yrecheck-test", "Run basic rechecking (internal test only).")

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

@@ -480,6 +480,8 @@ object Contexts {
     /** Is the flexible types option set? */
     def flexibleTypes: Boolean = base.settings.YexplicitNulls.value && !base.settings.YnoFlexibleTypes.value
 
+    def nullifyTasty: Boolean = base.settings.YexplicitNulls.value && base.settings.YnullifyTasty.value
+
     /** Is the best-effort option set? */
     def isBestEffort: Boolean = base.settings.YbestEffort.value
 

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

@@ -137,6 +137,7 @@ object ImplicitNullInterop:
       else tp match
         case tp: TypeRef
           if !ctx.flexibleTypes && tp.isRef(defn.RepeatedParamClass) => false
+        case tp: TypeRef if defn.isTupleClass(tp.symbol) => false
         case _ => true
 
     override def apply(tp: Type): Type = tp match
@@ -193,7 +194,6 @@ object ImplicitNullInterop:
         val savedSkipCurrentLevel = skipCurrentLevel
         val savedSkipResultType = skipResultType
 
-        skipCurrentLevel = true
         val parent2 = this(tp.parent)
 
         skipCurrentLevel = false

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -921,10 +921,10 @@ class TreeUnpickler(reader: TastyReader,
 
       def ta = ctx.typeAssigner
 
-      // If explicit nulls is enabled, and the source file did not have explicit
+      // If explicit nulls and `YnullifyTasty` is enabled, and the source file did not have explicit
       // nulls enabled, nullify the member to allow for compatibility.
       def nullify(sym: Symbol) =
-        if (ctx.explicitNulls && ctx.flexibleTypes && !explicitNulls) then
+        if (ctx.nullifyTasty && !explicitNulls) then
           sym.info = ImplicitNullInterop.nullifyMember(sym, sym.info, sym.is(Enum))
 
       val name = readName()

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -217,7 +217,7 @@ class CompilationTests {
     locally {
       val unsafeFile = compileFile("tests/explicit-nulls/flexible-unpickle/neg/Unsafe_1.scala", explicitNullsOptions without "-Yexplicit-nulls")
       val flexibleFile = compileFile("tests/explicit-nulls/flexible-unpickle/neg/Flexible_2.scala",
-          explicitNullsOptions.withClasspath(defaultOutputDir + testGroup + "/Unsafe_1/neg/Unsafe_1"))
+          explicitNullsOptions.and("-Ynullify-tasty").withClasspath(defaultOutputDir + testGroup + "/Unsafe_1/neg/Unsafe_1"))
 
       flexibleFile.keepOutput.checkExpectedErrors()
 
@@ -237,8 +237,8 @@ class CompilationTests {
     locally {
       val tests = List(
         compileFile("tests/explicit-nulls/flexible-unpickle/pos/Unsafe_1.scala", explicitNullsOptions without "-Yexplicit-nulls"),
-        compileFile("tests/explicit-nulls/flexible-unpickle/pos/Flexible_2.scala", explicitNullsOptions.withClasspath(
-          defaultOutputDir + testGroup + "/Unsafe_1/pos/Unsafe_1")),
+        compileFile("tests/explicit-nulls/flexible-unpickle/pos/Flexible_2.scala",
+        explicitNullsOptions.and("-Ynullify-tasty").withClasspath(defaultOutputDir + testGroup + "/Unsafe_1/pos/Unsafe_1")),
       ).map(_.keepOutput.checkCompile())
 
       tests.foreach(_.delete())