Expression produces a union type that is too complex to represent

Bug Report

🔎 Search Terms

"Expression produces a union type that is too complex to represent"
(found #33130 which seems unrelated as it has been fixed)

🕗 Version & Regression Information

• Present from 4.1.5 to 5.0.2 in playground.

⏯ Playground Link

Example in Playground

💻 Code

function computed<N extends keyof Longhands>(
  property: Longhands[N][1],
  specified: Longhands[N][0]
) {
  // error happens on this line
  property.compute(specified);
}

interface Property<T> {
  compute: (value: T) => T;
}

type Wrapper<T> = [T, Property<T>];

interface Longhands {
  "font-family": Wrapper<Family>;
  "font-size": Wrapper<Size>;
  "font-stretch": Wrapper<Stretch>;
  "font-variant-caps": Wrapper<Caps>;
  "font-variant-east-asian": Wrapper<EastAsian>;
  "font-variant-ligatures": Wrapper<Ligatures>;
  "font-variant-numeric": Wrapper<Numeric>;
  "font-variant-position": Wrapper<Position>;
  "font-weight": Wrapper<Weight>;
}

class Keyword<K extends string> {
  keyword: K;
  constructor(keyword: K) {
    this.keyword = keyword;
  }
}

type Family = Keyword<"serif">;
type Size = Keyword<"length">;
type Stretch =
  | Keyword<"percentage">
  | Keyword<"ultra-condensed">
  | Keyword<"extra-condensed">
  | Keyword<"condensed">
  | Keyword<"semi-condensed">
  | Keyword<"normal">
  | Keyword<"semi-expanded">
  | Keyword<"expanded">
  | Keyword<"extra-expanded">
  | Keyword<"ultra-expanded">;
type Caps = Keyword<"normal">;
type EastAsian =
  | Keyword<"normal">
  | Keyword<"jis78">
  | Keyword<"jis83">
  | Keyword<"jis90">
  | Keyword<"jis04">
  | Keyword<"simplified">
  | Keyword<"traditional">
  | Keyword<"proportional-width">
  | Keyword<"full-width">
  | Keyword<"ruby">;
type Ligatures =
  | Keyword<"none">
  | Keyword<"normal">
  | Keyword<"common-ligatures">
  | Keyword<"no-common-ligatures">
  | Keyword<"discretionary-ligatures">
  | Keyword<"no-discretionary-ligatures">
  | Keyword<"historical-ligatures">
  | Keyword<"no-historical-ligatures">
  | Keyword<"contextual">
  | Keyword<"no-contextual">;
type Numeric =
  | Keyword<"normal">
  | Keyword<"lining-nums">
  | Keyword<"oldstyle-nums">
  | Keyword<"proportional-nums">
  | Keyword<"tabular-nums">
  | Keyword<"diagonal-fractions">
  | Keyword<"stacked-fractions">
  | Keyword<"ordinal">
  | Keyword<"slashed-zero">;
type Position = Keyword<"normal"> | Keyword<"sub"> | Keyword<"super">;
type Weight =
  | Keyword<"number">
  | Keyword<"normal">
  | Keyword<"bold">
  | Keyword<"bolder">
  | Keyword<"lighter">;

Note: interestingly, I detected the error in the much more complex real code of Alfa where it only triggers in TS 4.9.3, but not in 4.8.4 🤔 I am not sure which simplification happens in my real code to keep the union smaller…

🙁 Actual behavior

The code produces the error:

Expression produces a union type that is too complex to represent

🙂 Expected behavior

The union should be around 50 items big, far below the limit.