Comparison of union types should error

TypeScript Version: v4.0.2

Search Terms: union type comparison

Code

function compare(a: number | string, b: number | string): boolean {
  return a < b; // <- Fails to error
}
compare(1, "a");
1 < "a"; // Correctly errors: Operator '<' cannot be applied to types 'number' and 'string'.

This is a simplified example taken from an error found in code review. The function's type signature should really be: function compare<T extends number | string>(a: T, b: T): boolean and I would have expected TypeScript to error for the uncorrected type signature.

Expected behavior:

Typescript should catch the error comparing union types within the function as Flow (v0.136.0) does: flow.org/try example.

Actual behavior:

Typescript does not catch the error for a < b.

Playground Link: example in typescriptlang.org/play.

Related Issues:

Riproduci il confronto segnalato nel TypeScript Playground e confrontalo con il confronto diretto tra number/string mostrato nell’issue. Traccia il modo in cui il type checker gestisce gli operatori relazionali per gli operandi di tipo union, quindi aggiungi un test di regressione che verifichi la diagnostica prevista e verifica che i confronti validi tra valori dello stesso tipo continuino a funzionare.