TypeScript: support optional and rest elements in static tuple type

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

@@ -2166,7 +2166,7 @@ class TupleType extends ArrayType, @tupletype {
   }
 
   /**
-   * Gets the number of elements in this tuple type.
+   * Gets the number of elements in this tuple type, including optional elements and the rest element.
    */
   int getNumElementType() {
     result = count(int i | exists(getElementType(i)))
@@ -2181,6 +2181,31 @@ class TupleType extends ArrayType, @tupletype {
   PlainArrayType getUnderlyingArrayType() {
     result.getArrayElementType() = getArrayElementType()
   }
+
+  /**
+   * Gets the number of required tuple elements, that is, excluding optional and rest elements.
+   *
+   * For example, the minimum length of `[number, string?, ...number[]]` is 1.
+   */
+  int getMinimumLength() {
+    tuple_type_min_length(this, result)
+  }
+
+  /**
+   * Holds if this tuple type ends with a rest element, such as `[number, ...string[]]`.
+   */
+  predicate hasRestElement() {
+    tuple_type_rest(this)
+  }
+
+  /**
+   * Gets the type of the rest element, if there is one.
+   *
+   * For example, the rest element of `[number, ...string[]]` is `string`.
+   */
+  Type getRestElementType() {
+    hasRestElement() and result = getElementType(getNumElementType() - 1)
+  }
 }
 
 /**

javascript/ql/src/semmlecode.javascript.dbscheme

@@ -738,6 +738,15 @@ self_types(
   int selfType: @typereference ref
 );
 
+tuple_type_min_length(
+  unique int typ: @type ref,
+  int minLength: int ref
+);
+
+tuple_type_rest(
+  unique int typ: @type ref
+);
+
 // comments
 comments (unique int id: @comment,
           int kind: int ref,

javascript/ql/test/library-tests/TypeScript/Types/TupleTypes.expected

@@ -0,0 +1,10 @@
+| tst.ts:34:5:34:9 | tuple | [number, string] | 0 | number | 2 | no-rest |
+| tst.ts:34:5:34:9 | tuple | [number, string] | 1 | string | 2 | no-rest |
+| tst.ts:36:5:36:28 | tupleWi ... Element | [number, string, number?] | 0 | number | 2 | no-rest |
+| tst.ts:36:5:36:28 | tupleWi ... Element | [number, string, number?] | 1 | string | 2 | no-rest |
+| tst.ts:36:5:36:28 | tupleWi ... Element | [number, string, number?] | 2 | number | 2 | no-rest |
+| tst.ts:38:5:38:24 | tupleWithRestElement | [number, ...string[]] | 0 | number | 1 | string |
+| tst.ts:38:5:38:24 | tupleWithRestElement | [number, ...string[]] | 1 | string | 1 | string |
+| tst.ts:39:5:39:36 | tupleWi ... lements | [number, string?, ...number[]] | 0 | number | 1 | number |
+| tst.ts:39:5:39:36 | tupleWi ... lements | [number, string?, ...number[]] | 1 | string | 1 | number |
+| tst.ts:39:5:39:36 | tupleWi ... lements | [number, string?, ...number[]] | 2 | number | 1 | number |

javascript/ql/test/library-tests/TypeScript/Types/TupleTypes.ql

@@ -0,0 +1,12 @@
+import javascript
+
+string getRest(TupleType tuple) {
+  if tuple.hasRestElement() then
+    result = tuple.getRestElementType().toString()
+  else
+    result = "no-rest"
+}
+
+from Expr e, TupleType tuple, int n
+where e.getType() = tuple
+select e, tuple, n, tuple.getElementType(n), tuple.getMinimumLength(), getRest(tuple)