fix(typescript): restore support for typescript 3

definitelyTypedTests/package.json

@@ -16,10 +16,10 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
+    "cross-var": "^1.1.0",
     "dotenv": "^8.2.0",
     "ttypescript": "^1.5.10",
-    "typescript": "^4.0.2",
-    "upath": "^1.2.0",
-    "cross-var": "^1.1.0"
+    "typescript": "^3.9.7",
+    "upath": "^1.2.0"
   }
 }

package.json

@@ -84,7 +84,7 @@
         "ts-node": "^9.0.0",
         "tsconfig-paths": "^3.9.0",
         "ttypescript": "1.5.10",
-        "typescript": "^4.0.2",
+        "typescript": "^3.9.7",
         "webpack": "^4.44.1",
         "webpack-cli": "^3.3.12",
         "webpack-merge": "^5.1.2",

src/transformer/descriptor/tuple/tuple.ts

@@ -7,6 +7,21 @@ export function GetTupleDescriptor(
   scope: Scope
 ): ts.Expression {
   return ts.createArrayLiteral(
-    node.elements.map((e: ts.TypeNode) => GetDescriptor(e, scope))
+    getDescriptorFromTypeNodes(
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      ((node as any).elementTypes as ts.NodeArray<ts.TypeNode>) ||
+        // Typescript 4 doesn't have elementTypes but it does have elements, to support both versions
+        // this casting is needed.
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        ((node as any).elements as ts.NodeArray<ts.TypeNode>),
+      scope
+    )
   );
 }
+
+function getDescriptorFromTypeNodes(
+  nodes: ts.NodeArray<ts.TypeNode>,
+  scope: Scope
+): ts.Expression[] {
+  return nodes.map((e: ts.TypeNode) => GetDescriptor(e, scope));
+}

src/transformer/mockDefiner/mockDefiner.ts

@@ -182,9 +182,8 @@ export class MockDefiner {
       );
     }
 
-    // NOTE: TypeScript does not support inference through has/get, but we know
-    // for a fact that the result here is a string!
-    return this._declarationCache.get(declaration) as string;
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    return this._declarationCache.get(declaration)!;
   }
 
   public registerMockFor(