Right now the parent property is optional on all AST nodes.
However in reality, it's always defined on all AST nodes, except for Program, where it is missing.

This leads to a bunch of unnecessary ifs/optional chains/"trust me it's right" assertions, all of which "harm" the type safety, or are impossible to get coverage on (leading to istanbul ignore statements):

// this shouldn't happen, checking just in case
/* istanbul ignore if */ if (node.parent) {
  // ...
}

// selector asserts this is correct
const parent = node.parent as TSESTree.VariableDeclaration;

if (node.parent?.type === AST_NODE_TYPES.VariableDeclaration) {
  // ...
}

There is an issue with this, however.
typescript-estree does not assign the parent property; this is done by ESLint as it traverses the AST at lint time.

So there are two solutions I can see:

1. change typescript-estree so that it sets the parent appropriately.
2. create a separate set of types for use within plugins, which correctly type the parent property.

As an aside, doing this would unlock a really awesome benefit wherein we could make all the types generic, allowing you to specify the expected parent type ahead of time:

interface BaseNode<TParent extends Node = Node> {
  // ...
  parent: TParent;
}

interface VariableDeclarator<TParent extends Node = Node> extends BaseNode<TParent> {}

interface Program extends BaseNode {
  parent?: never;
}

This would be great for things like return values of functions which explicitly check parent types, and for eslint selectors that explicitly state the tree structure (VariableDeclaration > VariableDeclarator).