Function decorateMethod

Helper for creating hybrid legacy/TC39 decorator/wrapper functions, e.g.:

// As wrapper function
export function myDecorator<T>(fn: SomeFnType<T>): SomeFnType<T>;

// TC39 decorator
export function myDecorator<T>(fn: SomeFnType<T>, ctx: {kind: "method"}): SomeFnType<T>

// "Legacy"/"TypeScript Experimental" Decorator
export function myDecorator<T,D extends {value?: SomeFnType<T>}>(clsOrProto:any, name: string|symbol, desc: D): D

// Implementation
export function myDecorator<T>(factory: SomeFnType<T>, ...args: any[]): SomeFnType<T> {
    // extra args means we're being used as a decorator, so run as decorator:
    if (args.length) return decorateMethod(myDecorator, factory, ...args as [any, any]);
    // No extra args, we're b
    return () => {
    }
}
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Yes, this is fairly ugly, but it's also the only way to make this work when the wrapper is a generic function. (TypeScript's type system doesn't allow generic values, declaring functions as implementing interfaces, higher-order kinds, or any other tricks that would let you avoid this giant ball of boilerplate for generic decorators.)