Wrap unsafeNull members in flexible types during unpickling

Author: HarrisL2

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

@@ -55,7 +55,7 @@ object JavaNullInterop {
    */
   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")
+    // assert(sym.is(JavaDefined), "can only nullify java-defined members")
 
     // Some special cases when nullifying the type
     if isEnumValueDef || sym.name == nme.TYPE_ then

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

@@ -981,6 +981,12 @@ class TreeUnpickler(reader: TastyReader,
           sym.info = tpt.tpe
           ValDef(tpt)
       }
+
+      // If explicit nulls is enabled, and the source file did not have explicit
+      // nulls enabled, nullify the member to allow for compatibility.
+      if (ctx.explicitNulls && !explicitNulls) then
+        sym.info = JavaNullInterop.nullifyMember(sym, sym.info, sym.is(Enum))
+
       goto(end)
       setSpan(start, tree)
 

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

@@ -215,8 +215,18 @@ class CompilationTests {
       compileFilesInDir("tests/explicit-nulls/pos", explicitNullsOptions),
       compileFilesInDir("tests/explicit-nulls/flexible-types-common", explicitNullsOptions),
       compileFilesInDir("tests/explicit-nulls/unsafe-common", explicitNullsOptions and "-language:unsafeNulls" and "-Yno-flexible-types"),
-    )
-  }.checkCompile()
+    ).checkCompile()
+
+    locally {
+      val tests = List(
+        compileFile("tests/explicit-nulls/flexible-unpickle/Unsafe_1.scala", explicitNullsOptions without "-Yexplicit-nulls"),
+        compileFile("tests/explicit-nulls/flexible-unpickle/Flexible_2.scala", explicitNullsOptions.withClasspath(
+          defaultOutputDir + testGroup + "/Unsafe_1/flexible-unpickle/Unsafe_1")),
+      ).map(_.keepOutput.checkCompile())
+
+      tests.foreach(_.delete())
+    }
+  }
 
   @Test def explicitNullsWarn: Unit = {
     implicit val testGroup: TestGroup = TestGroup("explicitNullsWarn")

tests/explicit-nulls/flexible-unpickle/Flexible_2.scala

@@ -0,0 +1,7 @@
+@main
+def Flexible_2() =
+  val s2: String | Null = "foo"
+  val unsafe = new Unsafe_1()
+  val s: String = unsafe.foo(s2)
+  unsafe.foo("")
+  unsafe.foo(null)

tests/explicit-nulls/flexible-unpickle/Unsafe_1.scala

@@ -0,0 +1,6 @@
+class Unsafe_1 {
+  def foo(s: String): String = {
+    if (s == null) then "nullString"
+    else s
+  }
+}