export function withValidate<C extends t.Any>(codec: C, validate: C['validate'], name: string = codec.name): C { ... }

import { withValidate } from 'io-ts-types/lib/withValidate'
import * as t from 'io-ts'
import { PathReporter } from 'io-ts/lib/PathReporter'
import { either, right } from 'fp-ts/lib/Either'

const T = withValidate(t.number, (u, c) => either.map(t.number.validate(u, c), n => n * 2))

assert.deepStrictEqual(T.decode(1), right(2))
assert.deepStrictEqual(PathReporter.report(T.decode(null)), ['Invalid value null supplied to : number'])