Call chains: receiver data flow info is dropped for extensions with nullable receiver #KT-10056 Fixed

compiler/testData/diagnostics/tests/smartCasts/safecalls/nullableReceiverWithFlexible.kt

@@ -0,0 +1,27 @@
+// FILE: Foo.java
+public class Foo {
+    public String bar;
+
+    private Foo(String bar) {
+        this.bar = bar;
+    }
+
+    public static Foo create(String bar) {
+        return new Foo(bar);
+    }
+}
+
+// FILE: Test.kt
+public inline fun <T, R> T.let(f: (T) -> R): R = f(this)
+
+fun test() {
+    val foo = Foo.create(null)
+    foo?.bar.let {
+        // Error, foo?.bar is nullable
+        it<!UNSAFE_CALL!>.<!>length
+        // Foo is nullable but flexible, so call is considered safe here
+        foo.bar.length
+        // Correct
+        foo?.bar?.length
+    }
+}