Code cleanup: isIntersectionType & similar internally

[JoshuaKGoldberg]

Suggestion

🔍 Search Terms

isIntersectionType type predicate internal source code

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Spinning #49727 (comment) into its own issue: there are a lot of type assertions internal to TypeScript that seem to predate type predicates.

It'd be nice to test out this this kind of change to the internal TypeScript source code:

- if (type.flags & TypeFlags.Intersection) {
+ if (isIntersectionType(type)) {
-     for (const subType of (type as IntersectionType).types) {
+     for (const subType of type.types) {

📃 Motivating Example

n/a

💻 Use Cases

See changes in #49727.

Edit: ...and see #50010 for a draft PR.

[JoshuaKGoldberg]

Per #50010 (comment), perf is hurt by this 😢.