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:

Reproduziere den gemeldeten Vergleich im TypeScript Playground und vergleiche ihn mit dem im Issue gezeigten direkten number/string-Vergleich. Verfolge, wie der Type Checker relationale Operatoren für Union-Operanden verarbeitet, füge anschließend einen Regressionstest hinzu, der die erwartete Diagnose überprüft, und verifiziere, dass gültige Vergleiche desselben Typs weiterhin funktionieren.