Provide a way to indicate that a function declaration conforms to some interface/type

[dinofx]

When using jsdoc, you can do something like:

/**
 * @typedef {(path: string[], line: number) => void} PathHandler
 */

/** @type {PathHandler} */
function handlePath(path, lineNumber) {
  //TODO
}

This indicates that the type of handlePath is determined by PathHandler. The types for params path and lineNumber are resolved accordingly.

When converting this code to typescript, there is no way to associate the function declaration with the new type/interface. One workaround is just to repeat all of the type information from PathHandler at every function that conforms to the type. Another workaround is to use a function expression, like const handlePath: PathHandler = function(....), but this loses hoisting and unnecessarily imposes one style over another.

There should be a way to type the entire function declaration similar to the jsdoc equivalent, e.g.:

function:PathHandler handlePath(path, lineNumber) {
  //TODO
}

[RyanCavanaugh]

Duplicate #22063

[typescript-bot]

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