Synthetic extensions wins against top-level extension.

compiler/testData/diagnostics/tests/resolve/priority/syntheticPropertiesVsExtensions.kt

@@ -0,0 +1,31 @@
+// !CHECK_TYPE
+// !DIAGNOSTICS: -UNUSED_EXPRESSION -UNUSED_PARAMETER -UNUSED_VARIABLE
+
+// FILE: A.java
+public class A {
+    public A getFoo() { return 3; }
+}
+
+// FILE: 1.kt
+
+private val A.foo: Int get() = 4
+
+fun test(a: A) {
+    a.foo checkType { _<A>() }
+    with(a) {
+        foo checkType { _<A>() }
+    }
+}
+
+class B {
+    private val A.foo: B get() = this@B
+
+    fun test(a: A) {
+        a.foo checkType { _<B>() } // todo
+        with(a) {
+            foo checkType { _<B>() } // todo
+        }
+    }
+}
+
+fun <T, R> with(receiver: T, f: T.() -> R): R = receiver.f()