stylesheet-validator.ts

type CSSProperty = {
    width: string
    height: string
}
// Placeholder
type Theme = string

type StylesheetGroup = {
    [s: string]: CSSProperty
}

type StylesheetSchema = ((...args: any[]) => StylesheetGroup) | StylesheetGroup

type ValidStylesheetSchema<T> = ({
    [K1 in keyof T]: (T[K1] extends CSSProperty ? true : false) | {
        [K2 in keyof T[K1]]: K2 extends keyof CSSProperty ? true : false
    }[keyof T[K1]]
}[keyof T])

type StylesheetValidator<T extends StylesheetSchema> =
    T extends (...args: any[]) => infer P ? (ValidStylesheetSchema<P> extends true ? T : "not a valid stylesheet") :
    T extends StylesheetSchema ? (ValidStylesheetSchema<T> extends true ? T : "not a valid stylesheet") :
    "not a valid stylesheet"

function stylesheet<T extends StylesheetValidator<P>, P extends StylesheetSchema = T>(value: T): T {
   return value;
}

const styles = stylesheet((theme: Theme) => ({
    a:  {
        width: "12",
        height: "14"
    }
}))

const styles2 = stylesheet({
    a:  {
        width: "12",
        height: "14"
    }
}))

type Many = 'foo' | 'bar' | 'baz'