Add regression test

Author: ahejlsberg

tests/cases/conformance/types/typeRelationships/typeInference/bivariantInferences.ts

@@ -0,0 +1,12 @@
+// @strict: true
+
+// Repro from #27337
+
+interface Array<T> {
+    equalsShallow<T>(this: ReadonlyArray<T>, other: ReadonlyArray<T>): boolean;
+}
+
+declare const a: (string | number)[] | null[] | undefined[] | {}[];
+declare const b: (string | number)[] | null[] | undefined[] | {}[];
+
+let x = a.equalsShallow(b);