TypeScript v5 missing typeArguments with default generics

[Shinigami92]

Search terms

• typeArguments
• generics
• TypeScript v5

Expected Behavior

We have defined a type:

// ./types.ts
export type LiteralUnion<T extends U, U = string> =
  | T
  | (U & { zz_IGNORE_ME?: never });

// ./index.ts
export type Test = LiteralUnion<'a' | 'b'>;

TypeScript v4.x produced

type.typeArguments[0] -> "'a' | 'b'"
type.typeArguments[1] -> "string"

Actual Behavior

TypeScript v5.x produced

type.typeArguments[0] -> "'a' | 'b'"

Steps to reproduce the bug

Repro: TypeStrong/typedoc-repros#27

Relevant issue: faker-js/faker#1953

Environment

• Typedoc version: 0.24.0-beta.7 | 0.23
• TypeScript version: v5.0.2
• Node.js version: 18
• OS: mac

[Gerrit0]

TypeDoc is converting what we get from TypeScript correctly, and this is an intentional change in TypeScript 5.0 - microsoft/TypeScript#51771.

TypeScript still has some work to do to be consistent about when defaulted type parameters are included (see Anders' follow up comment), but from some discussion with Jake Bailey, they're far more likely to move towards not eagerly expanding alias lists than revert this change (see also microsoft/TypeScript#52670)