Version 5.0.4 -> 5.1.3 Regression when using generic types

[chentsulin]

Bug Report

🔎 Search Terms

Generic, 5.1.3

🕗 Version & Regression Information

• This changed between versions 5.0.4 and 5.1.3

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.1.3#code/C4TwDgpgBAMhAeBLAxgQwDYGED2BbXqAdgCYA8AKgAqojrarEB8UAvFAEoTLYBOZAzsB6JCAcwA0UQhABuEHowDcAKGWhIUAJKF+84OQTBqtesVYcIDbIXQhSAbyjBDALiiDhYqAF8lqgGYAroTIwIjWUMg8ls44+ERk5IwAFACUbnBIaFh4BCQUzPbKUCVQ0cCBPIRQ9t4q3qrcOsBaAHIAygCi7OQA+uSdABp9mADyALLjAIKtACIZCCgYcXlk2ro8+obGdAzMLMWR0aixuQlpikA

💻 Code

type LexicalCommand<TPayload> = Record<string, never>;

type InsertTextPayload = Readonly<{ text: string }>;

function createCommand<T>(): LexicalCommand<T> {
    return {};
}

const INSERT_TEXT_COMMAND: LexicalCommand<InsertTextPayload> =
  createCommand();

🙁 Actual behavior

The type of INSERT_TEXT_COMMAND is LexicalCommand<TPayload> in 5.1.3 (but LexicalCommand<InsertTextPayload> in 5.0.4).

🙂 Expected behavior

The type of INSERT_TEXT_COMMAND should be LexicalCommand<InsertTextPayload>.

[fatcerberus]

I can't say why the behavior changed, but this code is fundamentally problematic: LexicalCommand doesn't use its type parameter so it doesn't matter what type you give for TPayload (all Record<string, never> are equivalent). TS tends to misbehave around phantom types because generic inference is structural¹. If you actually use TPayload in the type then you get the expected inference in all TS versions.

Footnotes

1. It's like solving for x in 0 * x = 0; any value is a valid solution. ↩

[RyanCavanaugh]

The report as written is benign, but the declaration emit is messed up. If the input is modified to remove the type annotation:

type LexicalCommand<TPayload> = Record<string, never>;

type InsertTextPayload = Readonly<{ text: string }>;

function createCommand<T>(): LexicalCommand<T> {
    return {};
}

const INSERT_TEXT_COMMAND = createCommand();

then the output is

type LexicalCommand<TPayload> = Record<string, never>;
type InsertTextPayload = Readonly<{
    text: string;
}>;
declare function createCommand<T>(): LexicalCommand<T>;
declare const INSERT_TEXT_COMMAND: LexicalCommand<TPayload>;

which refers to the unbound TPayload