type1.ts

export const fooPath = 'aa/:id/:foo/:bar'

type AllMethodPathAndParams = {
  [fooPath]: {
    GET: { id: number }
    POST: undefined
  }
}

function f<M extends 'GET' | 'POST', K extends keyof AllMethodPathAndParams>(
  method: M,
  path: K,
  params: Exclude<AllMethodPathAndParams[K][M], undefined>
) {
  console.log(method, path, params)
}

f('GET', fooPath, { id: 2 })

type2.ts

type PathSpec = string | { path: any; spec: any }
type ApiInfoBase = {
  pathSpec: PathSpec[]
}
type Verbs = 'GET' | 'POST' | 'DELETE'
type ApiInfo = {
  pathSpec: PathSpec[]
  _paramType?: { [key in Verbs]?: any }
}
export const fooPath = {
  pathSpec: ['/a', '/b', 'etc']
} as ApiInfoBase & { _paramType?: { GET: { id: number } } }
export const barPath = {
  verb: 'GET' as const,
  pathSpec: ['/a', 'etc']
} as ApiInfoBase & { _paramType?: { GET: { id: number }; POST: { foo: string } } }

function f<V extends Verbs, T extends ApiInfo>(
  method: V,
  info: T,
  params: Exclude<Exclude<T['_paramType'], undefined>[V], undefined>
) {
  console.log(method, info, params)
}

f('POST', barPath, { foo: 'a' })