Suggestion: short-circuit type inference for ternary operators when both branches have the same type

From https://github.com/squidfunk/typescript-issue-38339-repro (courtesy of @squidfunk)

TypeScript Version: 3.9.1-rc

Search Terms:

Subtype reduction, ternary

Code

import { JSX } from "preact"

type IntrinsicElements = JSX.IntrinsicElements[keyof JSX.IntrinsicElements]

export type IntrinsicElements2<
  T = IntrinsicElements
> = {
  [K in keyof T]: K extends "accept"
    ? T[K] | [string]
    : T[K]
}

export type IntrinsicElements3<
  T = IntrinsicElements2
> = {
  [K in keyof T]: K extends "class"
    ? T[K] | [boolean]
    : T[K]
}

export type IntrinsicElements4<
  T = IntrinsicElements3
> = {
  [K in keyof T]: K extends "id"
    ? T[K] | [number]
    : T[K]
}

export function foo(): IntrinsicElements4 | undefined {
  let t: IntrinsicElements4 | undefined
  // this takes 2 seconds to check
  return t
  // this takes 10 seconds to check
  // return true ? t : t
}

Expected behavior: return t and return true ? t : t check in a comparable amount of time

Actual behavior: the ternary is much slower

Playground Link: https://www.typescriptlang.org/play/?ts=Nightly#code/JYWwDg9gTgLgBAbzgKQMoA04F84DMoQhwBEYUApgIYDGMxAUPTAJ5jlwCSAdjFMFwGdg1AKIAbciHI8BcALwoMAOm69+Q0RKkyA2gGtyzCLkXoVPPoOHjJ0mAIC6jcgA9IsOCzacL661rsBACYAHno4OAAVeR81K01bGXoAPhiEcLgdAGk4fjgDIxNIhwAuOBzXGGkAE1liGmpyMDoMiIB+KOyHOAAfTIE4gHMnCIiyyK76LGc3aHgvdlVLDRttewBmMIjohSW-BLXglLSM7NyufMNjKNLyuEqauuoxSgEBBlG4Doms7r6dABGEAgEkoXBGo3Gk2m9Fc7nmrEWvniq0CABYtlEYnsUQEZOtjgp0hEznkCtdimUKi4qlxaiRgNUPqNvl1epkuABXEAA8hQCFjTq-KYzeF4TlcWjACAXXDAgAUAEoyjiVnj7Gj2RLquRcPxyNVEBkJPMVci1YkNVq6br9dUMgB6B0UGCcqAXeBOuAAMVeMAyLrdHqgnPYHXgZU9DrgqDEEAA7iKgA

Related Issues: #38339

リンクされた再現例と Playground の例から始め、提供された TypeScript コード内の単独の return と三項演算子による return を比較します。三項演算子の型チェック処理と関連する issue #38339 を調査します。両方の形式が期待される型を変更せず、同程度の時間でチェックされれば完了です。