Add tests

Author: ahejlsberg

tests/cases/conformance/types/unknown/unknownType1.ts

@@ -164,3 +164,20 @@ class C1 {
     b: unknown;
     c: any;
 }
+
+// Type parameter with explicit 'unknown' constraint not assignable to '{}'
+
+function f30<T, U extends unknown>(t: T, u: U) {
+    let x: {} = t;
+    let y: {} = u;
+}
+
+// Repro from #26796
+
+type Test1 = [unknown] extends [{}] ? true : false;  // false
+type IsDefinitelyDefined<T extends unknown> = [T] extends [{}] ? true : false;
+type Test2 = IsDefinitelyDefined<unknown>;  // false
+
+function oops<T extends unknown>(arg: T): {} {
+    return arg;  // Error
+}