Better intellisense for unions of tuple types

VSCode - Version 1.45.1
Typescript - Version 3.8.3

Search Terms

intellisense tuple

Suggestion

Have the suggestions of unions of tuple types only suggest valid things

Use Cases

I have a function which takes a 'path' to a property in a fixed object, say the object is like { a: { b: 1 } }, allowed paths are ["a"], or ["a", "b"].

I have some type which generates for me the union of allowed tuples, which is to be the type of the 'path'.

The fixed object the function takes paths in is very large and I don't want to have to remember the names of things, when I use the union, the intellisense correctly identifies if a path is invalid, but when using ctrl + space to get the suggestions, it suggests strings which if used are immediately afterwards recognised as invalid.

Examples

Consider a type like

type X = ["A"] | ["A", "B"]

Then if I write something like const x: X = [], when pressing ctrl + space in the array I would like to just be suggested "A" (in fact this is already the behaviour), after filling in "A", if I try to add another element it would just suggest "B", not "A", "B"

Checklist

My suggestion meets these guidelines:

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

No implementation file or test is named. Reproduce the tuple-union example in VS Code with the stated TypeScript versions, then identify the completion entry point involved; done means an array containing "A" suggests only "B" for its next element while preserving the existing validity checks.