Exempt combinations of Java fields and methods as double defs

Exempt combinations of Java fields and methods from being flagged
as double definitions.

Java fields don't get a getter, so the pattern

    int f
    int f() = f

is OK (and quite common).

Author: odersky

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -744,7 +744,10 @@ trait Checking {
     def checkDecl(decl: Symbol): Unit = {
       for (other <- seen(decl.name)) {
         typr.println(i"conflict? $decl $other")
-        if (decl.matches(other)) {
+        def javaFieldMethodPair =
+          decl.is(JavaDefined) && other.is(JavaDefined) &&
+          decl.is(Method) != other.is(Method)
+        if (decl.matches(other) && !javaFieldMethodPair) {
           def doubleDefError(decl: Symbol, other: Symbol): Unit =
             if (!decl.info.isErroneous && !other.info.isErroneous)
               ctx.error(DoubleDeclaration(decl, other), decl.sourcePos)

tests/pos/i4739.java

@@ -0,0 +1,6 @@
+public class TestJava {
+  int foo;
+  int foo() {
+    return this.foo;
+  }
+}