Support TypeScript 3.0's variable length tuples

Author: rpetrich

Diff for: test/programs/type-aliases-tuple-of-variable-length/main.ts

@@ -0,0 +1 @@
+export type MyTuple = [string, number, boolean?];

Diff for: test/programs/type-aliases-tuple-of-variable-length/schema.json

@@ -0,0 +1,29 @@
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "type": "array",
+    "items": [
+        {
+            "type": "string"
+        },
+        {
+            "type": "number"
+        },
+        {
+            "type": "boolean"
+        }
+    ],
+    "additionalItems": {
+        "anyOf": [
+            {
+                "type": "string"
+            },
+            {
+                "type": "number"
+            },
+            {
+                "type": "boolean"
+            }
+        ]
+    },
+    "minItems": 2
+}

Diff for: test/schema.test.ts

@@ -160,6 +160,7 @@ describe("schema", () => {
         assertSchema("type-aliases-recursive-anonymous", "MyAlias");
         assertSchema("type-aliases-recursive-export", "MyObject");
         */
+        assertSchema("type-aliases-tuple-of-variable-length", "MyTuple");
     });
 
     describe("enums", () => {

Diff for: typescript-json-schema.ts

@@ -378,7 +378,8 @@ export class JsonSchemaGenerator {
             const fixedTypes = elemTypes.map(elType => this.getTypeDefinition(elType as any));
             definition.type = "array";
             definition.items = fixedTypes;
-            definition.minItems = fixedTypes.length;
+            const targetTupleType = (propertyType as ts.TupleTypeReference).target;
+            definition.minItems = targetTupleType.minLength;
             definition.additionalItems = {
                 anyOf: fixedTypes
             };