Consider nullable annotations in explicit nulls

Fixes #21629

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

@@ -1125,6 +1125,11 @@ class Definitions {
     "reactor.util.annotation.NonNullApi" ::
     "io.reactivex.annotations.NonNull" :: Nil)
 
+  @tu lazy val NullableAnnots: List[ClassSymbol] = getClassesIfDefined(
+    "javax.annotation.Nullable" ::
+    "org.jetbrains.annotations.Nullable" ::
+    "org.jspecify.annotations.Nullable" :: Nil)
+
   // convenient one-parameter method types
   def methOfAny(tp: Type): MethodType = MethodType(List(AnyType), tp)
   def methOfAnyVal(tp: Type): MethodType = MethodType(List(AnyValType), tp)

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

@@ -66,23 +66,26 @@ object JavaNullInterop {
       nullifyExceptReturnType(tp)
     else
       // Otherwise, nullify everything
-      nullifyType(tp)
+      nullifyType(tp, explicitlyNullable = hasNullableAnnot(sym))
   }
 
   private def hasNotNullAnnot(sym: Symbol)(using Context): Boolean =
     ctx.definitions.NotNullAnnots.exists(nna => sym.unforcedAnnotation(nna).isDefined)
 
+  private def hasNullableAnnot(sym: Symbol)(using Context): Boolean =
+    ctx.definitions.NullableAnnots.exists(nna => sym.unforcedAnnotation(nna).isDefined)
+
   /** If tp is a MethodType, the parameters and the inside of return type are nullified,
    *  but the result return type is not nullable.
    *  If tp is a type of a field, the inside of the type is nullified,
    *  but the result type is not nullable.
    */
   private def nullifyExceptReturnType(tp: Type)(using Context): Type =
-    new JavaNullMap(outermostLevelAlreadyNullable = true)(tp)
+    new JavaNullMap(outermostLevelAlreadyNullable = true, explicitlyNullable = false)(tp)
 
   /** Nullifies a Java type by adding `| Null` in the relevant places. */
-  private def nullifyType(tp: Type)(using Context): Type =
-    new JavaNullMap(outermostLevelAlreadyNullable = false)(tp)
+  private def nullifyType(tp: Type, explicitlyNullable: Boolean = false)(using Context): Type =
+    new JavaNullMap(outermostLevelAlreadyNullable = false, explicitlyNullable)(tp)
 
   /** A type map that implements the nullification function on types. Given a Java-sourced type, this adds `| Null`
    *  in the right places to make the nulls explicit in Scala.
@@ -95,8 +98,17 @@ object JavaNullInterop {
    *                                       This is useful for e.g. constructors, and also so that `A & B` is nullified
    *                                       to `(A & B) | Null`, instead of `(A | Null & B | Null) | Null`.
    */
-  private class JavaNullMap(var outermostLevelAlreadyNullable: Boolean)(using Context) extends TypeMap {
-    def nullify(tp: Type): Type = if ctx.flexibleTypes then FlexibleType(tp) else OrNull(tp)
+  private class JavaNullMap(var outermostLevelAlreadyNullable: Boolean, explicitlyNullable: Boolean)(using Context) extends TypeMap {
+    def nullify(tp: Type): Type = 
+      if ctx.flexibleTypes then {
+        if explicitlyNullable then {
+          OrNull(tp)
+        } else {
+          FlexibleType(tp)
+        }
+      } else {
+        OrNull(tp)
+      }
 
     /** Should we nullify `tp` at the outermost level? */
     def needsNull(tp: Type): Boolean =

tests/explicit-nulls/neg/i21629/J.java

@@ -0,0 +1,50 @@
+package javax.annotation;
+import java.util.*;
+
+public class J {
+
+    private static String getK() {
+        return "k";
+    }
+
+    @Nullable
+    public static final String k = getK();
+
+    @Nullable
+    public static String l = "l";
+
+    @Nullable
+    public final String m = null;
+
+    @Nullable
+    public String n = "n";
+
+    @Nullable
+    public static final String f(int i) {
+        return "f: " + i;
+    }
+
+    @Nullable
+    public static String g(int i) {
+        return "g: " + i;
+    }
+
+    @Nullable
+    public String h(int i) {
+        return "h: " + i;
+    }
+
+    @Nullable
+    public <T> String[] genericf(T a) {
+        String[] as = new String[1];
+        as[0] = "" + a;
+        return as;
+    }
+
+    @Nullable
+    public <T> List<T> genericg(T a) {
+        List<T> as = new ArrayList<T>();
+        as.add(a);
+        return as;
+    }
+}

tests/explicit-nulls/neg/i21629/Nullable.java

@@ -0,0 +1,8 @@
+package javax.annotation;
+
+import java.lang.annotation.*;
+
+// A "fake" Nullable Annotation for jsr305
+@Retention(value = RetentionPolicy.RUNTIME)
+@interface Nullable {
+}

tests/explicit-nulls/neg/i21629/S.scala

@@ -0,0 +1,15 @@
+// Test that Nullable annotations are working in Java files.
+
+import javax.annotation.J
+
+class S_3 {
+  def kk: String = J.k // error
+  def ll: String = J.l // error
+  def mm: String = (new J).m // error
+  def nn: String = (new J).n // error
+  def ff(i: Int): String = J.f(i) // error
+  def gg(i: Int): String = J.g(i) // error
+  def hh(i: Int): String = (new J).h(i) // error
+  def genericff(a: String | Null): Array[String | Null] = (new J).genericf(a) // error
+  def genericgg(a: String | Null): java.util.List[String] = (new J).genericg(a) // error
+}