feat(jsii): detect changing visibility when overriding (#1876)

Since C# does not allow changing the visibility of a member when
overriding it, added a check to ensure nobody attempts to make a
protected member public when overriding it.

This is a somewhat common pitfall for users (as seen for example in
aws/aws-cdk#9616), and the check will allow faster detection and
correction (i.e: at compilation time instead of during `jsii-pacmak`).

packages/jsii/lib/validator.ts

@@ -392,6 +392,14 @@ function _defaultValidations(): ValidationFunction[] {
       label: string,
       action: string,
     ) {
+      if (!!expected.protected !== !!actual.protected) {
+        const expVisibility = expected.protected ? 'protected' : 'public';
+        const actVisibility = actual.protected ? 'protected' : 'public';
+        diagnostic(
+          ts.DiagnosticCategory.Error,
+          `${label} changes visibility when ${action} (expected ${expVisibility}, found ${actVisibility})`,
+        );
+      }
       if (!deepEqual(actual.returns, expected.returns)) {
         const expType = spec.describeTypeReference(expected.returns?.type);
         const actType = spec.describeTypeReference(actual.returns?.type);
@@ -446,6 +454,14 @@ function _defaultValidations(): ValidationFunction[] {
       label: string,
       action: string,
     ) {
+      if (!!expected.protected !== !!actual.protected) {
+        const expVisibility = expected.protected ? 'protected' : 'public';
+        const actVisibility = actual.protected ? 'protected' : 'public';
+        diagnostic(
+          ts.DiagnosticCategory.Error,
+          `${label} changes visibility when ${action} (expected ${expVisibility}, found ${actVisibility})`,
+        );
+      }
       if (!deepEqual(expected.type, actual.type)) {
         const expType = spec.describeTypeReference(expected.type);
         const actType = spec.describeTypeReference(actual.type);

packages/jsii/test/negatives/neg.override-changes-visibility.ts

@@ -0,0 +1,20 @@
+// Note: not testing "public -> protected" because this is invalid TypeScript,
+//       so the type checker will already have caught it for us.
+
+export class BaseClass {
+  protected readonly property?: string;
+
+  protected method() {
+    return;
+  }
+}
+
+export class ChildClass extends BaseClass {
+  // This property cannot be public as it overrides the one on BaseClass which is protected
+  public readonly property?: string;
+
+  // This method cannot be public as it overrides the one on BaseClass which is protected
+  public method() {
+    throw new Error('Not Implemented');
+  }
+}