Syntax for hinting literal type inference #10195

Now that we have so many literal types we more than ever need new syntax that would make their use natural. Please consider the following:

const value = (true); // true
const value = true; // boolean

const value = ('a'); // 'a'
const value = 'a'; // string

const value = (1); // 1
const value = 1; // number

const value = ['a', 1]; // (string | number)[]
const value = (['a', 1]) // [string, number]
const value = ([('a'), (1)]) // ['a', 1];

Problem:

• There is no way to get a literal type value without explicit type annotation.

Solution:

• Add new syntax (or shall we say some new semantics to the old mostly unused syntax)

Highlights:

• Composable.
• Works fine with the existing syntax, doesn't need a new one.
• Highly unlikely to be a breaking change.
• Doesn't affect JavaScript semantics at all, can be executed as written and will work as expected.

Shortcomings:

• Undiscoverable syntax.
• Semantic conflicts (breaking changes) although rare:
  • generated code
  • conditional expressions
  • unintended excessive syntax

Prior work:

• Inspired by consider syntax for tuple literals: (['a', 2]) // [string, number] #9217 which is closed in favor of this one.