diff --git a/tests/baselines/reference/unionTypesAssignability.errors.txt b/tests/baselines/reference/unionTypesAssignability.errors.txt
new file mode 100644
index 00000000000..b32f71a38c1
--- /dev/null
+++ b/tests/baselines/reference/unionTypesAssignability.errors.txt
@@ -0,0 +1,154 @@
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(18,1): error TS2323: Type 'E' is not assignable to type 'D'.
+  Property 'foo1' is missing in type 'E'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(19,1): error TS2323: Type 'D | E' is not assignable to type 'D'.
+  Type 'E' is not assignable to type 'D'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(20,1): error TS2323: Type 'D' is not assignable to type 'E'.
+  Property 'foo2' is missing in type 'D'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(22,1): error TS2323: Type 'D | E' is not assignable to type 'E'.
+  Type 'D' is not assignable to type 'E'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(24,1): error TS2323: Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(25,1): error TS2323: Type 'string | number' is not assignable to type 'number'.
+  Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(26,1): error TS2323: Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(28,1): error TS2323: Type 'string | number' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(31,1): error TS2323: Type 'C' is not assignable to type 'D'.
+  Property 'foo1' is missing in type 'C'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(32,1): error TS2323: Type 'C' is not assignable to type 'E'.
+  Property 'foo2' is missing in type 'C'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(33,1): error TS2323: Type 'C' is not assignable to type 'D | E'.
+  Type 'C' is not assignable to type 'E'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(35,1): error TS2323: Type 'D' is not assignable to type 'E'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(37,1): error TS2323: Type 'E' is not assignable to type 'D'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(41,1): error TS2323: Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(43,1): error TS2323: Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(64,5): error TS2323: Type 'U' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(65,5): error TS2323: Type 'T' is not assignable to type 'U'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(69,5): error TS2323: Type 'T | U' is not assignable to type 'T'.
+  Type 'U' is not assignable to type 'T'.
+tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts(70,5): error TS2323: Type 'T | U' is not assignable to type 'U'.
+  Type 'T' is not assignable to type 'U'.
+
+
+==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts (19 errors) ====
+    var unionNumberString: number | string;
+    class C { }
+    class D extends C { foo1() { } }
+    class E extends C { foo2() { } }
+    var unionDE: D | E;
+    
+    var num: number;
+    var str: string;
+    var c: C;
+    var d: D;
+    var e: E;
+    
+    // A union type U is assignable to a type T if each type in U is assignable to T
+    c = d;
+    c = e;
+    c = unionDE; // ok
+    d = d;
+    d = e;
+    ~
+!!! error TS2323: Type 'E' is not assignable to type 'D'.
+!!! error TS2323:   Property 'foo1' is missing in type 'E'.
+    d = unionDE; // error e is not assignable to d
+    ~
+!!! error TS2323: Type 'D | E' is not assignable to type 'D'.
+!!! error TS2323:   Type 'E' is not assignable to type 'D'.
+    e = d;
+    ~
+!!! error TS2323: Type 'D' is not assignable to type 'E'.
+!!! error TS2323:   Property 'foo2' is missing in type 'D'.
+    e = e;
+    e = unionDE; // error d is not assignable to e
+    ~
+!!! error TS2323: Type 'D | E' is not assignable to type 'E'.
+!!! error TS2323:   Type 'D' is not assignable to type 'E'.
+    num = num;
+    num = str;
+    ~~~
+!!! error TS2323: Type 'string' is not assignable to type 'number'.
+    num = unionNumberString; // error string is not assignable to number
+    ~~~
+!!! error TS2323: Type 'string | number' is not assignable to type 'number'.
+!!! error TS2323:   Type 'string' is not assignable to type 'number'.
+    str = num;
+    ~~~
+!!! error TS2323: Type 'number' is not assignable to type 'string'.
+    str = str;
+    str = unionNumberString; // error since number is not assignable to string
+    ~~~
+!!! error TS2323: Type 'string | number' is not assignable to type 'string'.
+!!! error TS2323:   Type 'number' is not assignable to type 'string'.
+    
+    // A type T is assignable to a union type U if T is assignable to any type in U
+    d = c;
+    ~
+!!! error TS2323: Type 'C' is not assignable to type 'D'.
+!!! error TS2323:   Property 'foo1' is missing in type 'C'.
+    e = c;
+    ~
+!!! error TS2323: Type 'C' is not assignable to type 'E'.
+!!! error TS2323:   Property 'foo2' is missing in type 'C'.
+    unionDE = c; // error since C is not assinable to either D or E
+    ~~~~~~~
+!!! error TS2323: Type 'C' is not assignable to type 'D | E'.
+!!! error TS2323:   Type 'C' is not assignable to type 'E'.
+    d = d;
+    e = d;
+    ~
+!!! error TS2323: Type 'D' is not assignable to type 'E'.
+    unionDE = d; // ok
+    d = e;
+    ~
+!!! error TS2323: Type 'E' is not assignable to type 'D'.
+    e = e;
+    unionDE = e; // ok
+    num = num;
+    str = num;
+    ~~~
+!!! error TS2323: Type 'number' is not assignable to type 'string'.
+    unionNumberString = num; // ok 
+    num = str;
+    ~~~
+!!! error TS2323: Type 'string' is not assignable to type 'number'.
+    str = str;
+    unionNumberString = str; // ok
+    
+    // Any
+    var anyVar: any;
+    anyVar = unionDE;
+    anyVar = unionNumberString;
+    unionDE = anyVar;
+    unionNumberString = anyVar;
+    
+    // null
+    unionDE = null;
+    unionNumberString = null;
+    
+    // undefined
+    unionDE = undefined;
+    unionNumberString = undefined;
+    
+    // type parameters
+    function foo<T, U>(t: T, u: U) {
+        t = u; // error
+        ~
+!!! error TS2323: Type 'U' is not assignable to type 'T'.
+        u = t; // error
+        ~
+!!! error TS2323: Type 'T' is not assignable to type 'U'.
+        var x : T | U;
+        x = t; // ok
+        x = u; // ok
+        t = x; // error U not assignable to T
+        ~
+!!! error TS2323: Type 'T | U' is not assignable to type 'T'.
+!!! error TS2323:   Type 'U' is not assignable to type 'T'.
+        u = x; // error T not assignable to U
+        ~
+!!! error TS2323: Type 'T | U' is not assignable to type 'U'.
+!!! error TS2323:   Type 'T' is not assignable to type 'U'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/unionTypesAssignability.js b/tests/baselines/reference/unionTypesAssignability.js
new file mode 100644
index 00000000000..a67f23bb09f
--- /dev/null
+++ b/tests/baselines/reference/unionTypesAssignability.js
@@ -0,0 +1,165 @@
+//// [unionTypesAssignability.ts]
+var unionNumberString: number | string;
+class C { }
+class D extends C { foo1() { } }
+class E extends C { foo2() { } }
+var unionDE: D | E;
+
+var num: number;
+var str: string;
+var c: C;
+var d: D;
+var e: E;
+
+// A union type U is assignable to a type T if each type in U is assignable to T
+c = d;
+c = e;
+c = unionDE; // ok
+d = d;
+d = e;
+d = unionDE; // error e is not assignable to d
+e = d;
+e = e;
+e = unionDE; // error d is not assignable to e
+num = num;
+num = str;
+num = unionNumberString; // error string is not assignable to number
+str = num;
+str = str;
+str = unionNumberString; // error since number is not assignable to string
+
+// A type T is assignable to a union type U if T is assignable to any type in U
+d = c;
+e = c;
+unionDE = c; // error since C is not assinable to either D or E
+d = d;
+e = d;
+unionDE = d; // ok
+d = e;
+e = e;
+unionDE = e; // ok
+num = num;
+str = num;
+unionNumberString = num; // ok 
+num = str;
+str = str;
+unionNumberString = str; // ok
+
+// Any
+var anyVar: any;
+anyVar = unionDE;
+anyVar = unionNumberString;
+unionDE = anyVar;
+unionNumberString = anyVar;
+
+// null
+unionDE = null;
+unionNumberString = null;
+
+// undefined
+unionDE = undefined;
+unionNumberString = undefined;
+
+// type parameters
+function foo<T, U>(t: T, u: U) {
+    t = u; // error
+    u = t; // error
+    var x : T | U;
+    x = t; // ok
+    x = u; // ok
+    t = x; // error U not assignable to T
+    u = x; // error T not assignable to U
+}
+
+
+//// [unionTypesAssignability.js]
+var __extends = this.__extends || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    __.prototype = b.prototype;
+    d.prototype = new __();
+};
+var unionNumberString;
+var C = (function () {
+    function C() {
+    }
+    return C;
+})();
+var D = (function (_super) {
+    __extends(D, _super);
+    function D() {
+        _super.apply(this, arguments);
+    }
+    D.prototype.foo1 = function () {
+    };
+    return D;
+})(C);
+var E = (function (_super) {
+    __extends(E, _super);
+    function E() {
+        _super.apply(this, arguments);
+    }
+    E.prototype.foo2 = function () {
+    };
+    return E;
+})(C);
+var unionDE;
+var num;
+var str;
+var c;
+var d;
+var e;
+// A union type U is assignable to a type T if each type in U is assignable to T
+c = d;
+c = e;
+c = unionDE; // ok
+d = d;
+d = e;
+d = unionDE; // error e is not assignable to d
+e = d;
+e = e;
+e = unionDE; // error d is not assignable to e
+num = num;
+num = str;
+num = unionNumberString; // error string is not assignable to number
+str = num;
+str = str;
+str = unionNumberString; // error since number is not assignable to string
+// A type T is assignable to a union type U if T is assignable to any type in U
+d = c;
+e = c;
+unionDE = c; // error since C is not assinable to either D or E
+d = d;
+e = d;
+unionDE = d; // ok
+d = e;
+e = e;
+unionDE = e; // ok
+num = num;
+str = num;
+unionNumberString = num; // ok 
+num = str;
+str = str;
+unionNumberString = str; // ok
+// Any
+var anyVar;
+anyVar = unionDE;
+anyVar = unionNumberString;
+unionDE = anyVar;
+unionNumberString = anyVar;
+// null
+unionDE = null;
+unionNumberString = null;
+// undefined
+unionDE = undefined;
+unionNumberString = undefined;
+// type parameters
+function foo(t, u) {
+    t = u; // error
+    u = t; // error
+    var x;
+    x = t; // ok
+    x = u; // ok
+    t = x; // error U not assignable to T
+    u = x; // error T not assignable to U
+}
diff --git a/tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts b/tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts
new file mode 100644
index 00000000000..9ceee8845a5
--- /dev/null
+++ b/tests/cases/conformance/types/typeRelationships/assignmentCompatibility/unionTypesAssignability.ts
@@ -0,0 +1,71 @@
+var unionNumberString: number | string;
+class C { }
+class D extends C { foo1() { } }
+class E extends C { foo2() { } }
+var unionDE: D | E;
+
+var num: number;
+var str: string;
+var c: C;
+var d: D;
+var e: E;
+
+// A union type U is assignable to a type T if each type in U is assignable to T
+c = d;
+c = e;
+c = unionDE; // ok
+d = d;
+d = e;
+d = unionDE; // error e is not assignable to d
+e = d;
+e = e;
+e = unionDE; // error d is not assignable to e
+num = num;
+num = str;
+num = unionNumberString; // error string is not assignable to number
+str = num;
+str = str;
+str = unionNumberString; // error since number is not assignable to string
+
+// A type T is assignable to a union type U if T is assignable to any type in U
+d = c;
+e = c;
+unionDE = c; // error since C is not assinable to either D or E
+d = d;
+e = d;
+unionDE = d; // ok
+d = e;
+e = e;
+unionDE = e; // ok
+num = num;
+str = num;
+unionNumberString = num; // ok 
+num = str;
+str = str;
+unionNumberString = str; // ok
+
+// Any
+var anyVar: any;
+anyVar = unionDE;
+anyVar = unionNumberString;
+unionDE = anyVar;
+unionNumberString = anyVar;
+
+// null
+unionDE = null;
+unionNumberString = null;
+
+// undefined
+unionDE = undefined;
+unionNumberString = undefined;
+
+// type parameters
+function foo<T, U>(t: T, u: U) {
+    t = u; // error
+    u = t; // error
+    var x : T | U;
+    x = t; // ok
+    x = u; // ok
+    t = x; // error U not assignable to T
+    u = x; // error T not assignable to U
+}