TS: Unfold aliases in Type.unfold()

javascript/ql/src/semmle/javascript/TypeScript.qll

@@ -1816,7 +1816,7 @@ class Type extends @type {
    *
    * For example, for a type `(S & T) | U` this gets the types `S`, `T`, and `U`.
    */
-  Type unfold() {
+  Type unfoldUnionAndIntersection() {
     not result instanceof UnionOrIntersectionType and
     (
       result = this
@@ -1829,6 +1829,27 @@ class Type extends @type {
     )
   }
 
+  /**
+   * Repeatedly unfolds unions, intersections, and type aliases and gets any of the underlying types,
+   * or this type itself if it is not a union or intersection.
+   *
+   * For example, the type `(S & T) | U` unfolds to `S`, `T`, and `U`.
+   *
+   * If this is a type alias, the alias is itself included in the result, but this is not the case for intermediate type aliases.
+   * For example:
+   * ```js
+   * type One = number | string;
+   * type Two = One | Function & {x: string};
+   * One; // unfolds to number, string, and One
+   * Two; // unfolds to number, string, One, Function, {x: string}, and Two
+   * ```
+   */
+  Type unfold() {
+    result = unfoldUnionAndIntersection()
+    or
+    result = this.(TypeAliasReference).getAliasedType().unfoldUnionAndIntersection()
+  }
+
   /**
    * Holds if this refers to the given named type, or is declared as a subtype thereof,
    * or is a union or intersection containing such a type.

javascript/ql/test/library-tests/TypeScript/TypeAliases/TypeAliases.expected

@@ -3,15 +3,42 @@ rightHandSide
 | tst.ts:2:1:2:16 | type B<T> = T[]; | T[] |
 | tst.ts:8:10:8:20 | type C = A; | number |
 | tst.ts:15:1:15:23 | type Un ...  \| Two; | One \| Two |
+| tst.ts:17:1:17:36 | type Un ... mber }; | (One & { x: number; }) \| (Two & { x: number; }) |
+| tst.ts:18:1:18:21 | type Un ... Union2; | (One & { x: number; }) \| (Two & { x: number; }) |
+| tst.ts:19:1:19:21 | type Un ... Union3; | (One & { x: number; }) \| (Two & { x: number; }) |
+| tst.ts:20:1:20:30 | type Un ... number; | number \| (One & { x: number; }) \| (Two & { x: n... |
 getAliasedType
 | B<T> | T[] |
 | B<number> | number[] |
 | Union | One \| Two |
+| Union2 | (One & { x: number; }) \| (Two & { x: number; }) |
+| Union5 | number \| (One & { x: number; }) \| (Two & { x: n... |
 getTypeArgument
 | B<T> | 0 | T |
 | B<number> | 0 | number |
+unfold
+| B<T> | B<T> |
+| B<T> | T[] |
+| B<number> | B<number> |
+| B<number> | number[] |
+| Union | One |
+| Union | Two |
+| Union | Union |
+| Union2 | One |
+| Union2 | Two |
+| Union2 | Union2 |
+| Union2 | { x: number; } |
+| Union5 | One |
+| Union5 | Two |
+| Union5 | Union5 |
+| Union5 | number |
+| Union5 | { x: number; } |
 #select
 | tst.ts:1:1:1:16 | type A = number; | tst.ts:1:6:1:6 | A | 0 | tst.ts:1:10:1:15 | number |
 | tst.ts:2:1:2:16 | type B<T> = T[]; | tst.ts:2:6:2:6 | B | 1 | tst.ts:2:13:2:15 | T[] |
 | tst.ts:8:10:8:20 | type C = A; | tst.ts:8:15:8:15 | C | 0 | tst.ts:8:19:8:19 | A |
 | tst.ts:15:1:15:23 | type Un ...  \| Two; | tst.ts:15:6:15:10 | Union | 0 | tst.ts:15:14:15:22 | One \| Two |
+| tst.ts:17:1:17:36 | type Un ... mber }; | tst.ts:17:6:17:11 | Union2 | 0 | tst.ts:17:15:17:35 | Union & ... umber } |
+| tst.ts:18:1:18:21 | type Un ... Union2; | tst.ts:18:6:18:11 | Union3 | 0 | tst.ts:18:15:18:20 | Union2 |
+| tst.ts:19:1:19:21 | type Un ... Union3; | tst.ts:19:6:19:11 | Union4 | 0 | tst.ts:19:15:19:20 | Union3 |
+| tst.ts:20:1:20:30 | type Un ... number; | tst.ts:20:6:20:11 | Union5 | 0 | tst.ts:20:15:20:29 | Union4 \| number |

javascript/ql/test/library-tests/TypeScript/TypeAliases/TypeAliases.ql

@@ -14,3 +14,7 @@ query Type getAliasedType(TypeAliasReference ref) {
 query Type getTypeArgument(TypeAliasReference ref, int n) {
   result = ref.getTypeArgument(n)
 }
+
+query Type unfold(TypeAliasReference t) {
+  result = t.unfold()
+}

javascript/ql/test/library-tests/TypeScript/TypeAliases/tst.ts

@@ -13,3 +13,8 @@ interface One { a: number }
 interface Two { b: number }
 
 type Union = One | Two;
+
+type Union2 = Union & { x: number };
+type Union3 = Union2;
+type Union4 = Union3;
+type Union5 = Union4 | number;