highlight nullable methods that always return a non-null value (IDEA-166399)

java/java-tests/testData/inspection/dataFlow/fixture/NullableMethodReturningNotNull.java

@@ -0,0 +1,15 @@
+import org.jetbrains.annotations.*;
+
+final class Foo {
+  @<warning descr="@Nullable method 'foo' always return a non-null value">Nullable</warning> Object foo(int param) {
+    return param == 1 ? new Object() {
+      Object unrelated() {
+        return <warning descr="'null' is returned by the method which is not declared as @Nullable">null</warning>;
+      }
+    } : bar();
+  }
+
+  @NotNull Foo bar() {
+    return this;
+  }
+}