isTemplateDeclaration and Operation type narrowing.

[emi0x7d1]

We have the following code:

    for (const operation of namespace.operations.values()) {
      if (!isTemplateDeclaration(operation)) {
        this.emitter.emitType(operation);
      }
    }

Inside the if statement, operation gets narrowed to never. This happens because isTemplateDeclaration is defined as:

export function isTemplateDeclaration(
  type: TemplatedType
): type is TemplatedType & { node: TemplateDeclarationNode }

and Operation is defined as:

export interface Operation extends BaseType, DecoratedType, TemplatedTypeBase {
  // ...
  node: OperationStatementNode;
  // ...
}

and OperationStatementNode as:

export interface OperationStatementNode extends TemplateDeclarationNode, /* more base types */;

So, Operation always has a node with type TemplateDeclarationNode making the Operation always a template declaration. So, typescript correctly deduces that isTemplateDeclaration(operation) can never be false.

I wonder what's up here. Is that if statement safe to remove or is something wrong else where?

Thanks in advance.

[timotheeguerin]

I think there is maybe some tweaking we should do to the templated types so that they all have a templatedeclarationode and this intersection is not needed.

However for this specific case, I am assuming you are only looking to get template instance in that case I would use isTemplateInsance. For operations in particular there is a 3rd intermediate state(thought you wouldn't get it by iterating the namspace.operations). When you have a templated interface and a templated operation inside, if you instantiate the templated interface the templated operation are still not instance but not declaration either. Using isTemplateInstance will make sure the operation is actually an instance.