Fix #9392: Allow overriding a Java method when a same-named field is present

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

@@ -976,8 +976,22 @@ object Denotations {
         case FullMatch =>
           true
         case MethodNotAMethodMatch =>
-          // Java allows defining both a field and a zero-parameter method with the same name
-          !ctx.erasedTypes && !(symbol.is(JavaDefined) && other.symbol.is(JavaDefined))
+          !ctx.erasedTypes && {
+            val isJava = symbol.is(JavaDefined)
+            val otherIsJava = other.symbol.is(JavaDefined)
+            // A Scala zero-parameter method and a Scala non-method always match.
+            if !isJava && !otherIsJava then
+              true
+            // Java allows defining both a field and a zero-parameter method with the same name,
+            // so they must not match.
+            else if isJava && otherIsJava then
+              false
+            // A Java field never matches a Scala method.
+            else if isJava then
+              symbol.is(Method)
+            else // otherIsJava
+              other.symbol.is(Method)
+          }
         case ParamMatch =>
            // The signatures do not tell us enough to be sure about matching
           !ctx.erasedTypes && info.matches(other.info)

tests/neg/i9115/B.scala

@@ -1,3 +1,3 @@
 class B extends A {
   override val foo: String = "B" // error
-}
+}

tests/pos/i9392/J.java

@@ -0,0 +1,6 @@
+package pkg;
+
+public class J {
+    int i = 0;
+    public int i() { return 1; }
+}

tests/pos/i9392/S.scala

@@ -0,0 +1,20 @@
+class S1 extends pkg.J {
+  override def i(): Int = 2
+}
+
+// Unlike Scala 2 this doesn't compile, because this override of `i()`
+// also matches the non-overridable field `i`.
+// class S2 extends pkg.J {
+//   override def i: Int = 2
+// }
+
+object Test {
+  val s1 = new S1
+
+  val i1 = s1.i
+  val i2 = s1.i()
+
+  // val s2 = new S2
+
+  // val i3 = s2.i
+}