ts2367 Comparing non-comparable types not detected with optional property

TypeScript Version: 3.6.0-dev.20190730 (also with 3.5.1)

Search Terms: compare types, comparable, always false comparison, optional property, properties

Code

enum X {
  A= 'A'
}

function x(res: {action: string}):boolean {
  return res === X.A // error
}

function y(res: {action: string | undefined}):boolean {
  return res === X.A // error
}

function z(res: {action?: string}):boolean {
  return res === X.A // no error, but always false
}

function correct(res: {action: string}):boolean {
  return res.action === X.A // what the code should have been
}

Expected behavior:

The comparison in function z should have resulted in error TS2367: This condition will always return 'false' since the types '{ action?: string; }' and 'X.A' have no overlap.

Actual behavior:

Functions x, y give the correct error. Function z has no compiler error, but the condition will always evaluate to false since it is comparing an object and a string.

Playground Link: https://www.typescriptlang.org/play/#code/KYOwrgtgBAGlDeBYAUFNUCCBeKByDuKAviigGZggDGALgJYD2IUAHgBQBOwAzgFwIBDWoxD9uNDnRABzIgEpeAIwYMANsAHMkyLjTAdmXblCynYAOgxQA9NajAOHBh2KlkFavSZQAnpx788EJeolDikjJQAD5QlAAmwGRSwHHySirqmggouvqGPCZmMJY2dg5OLsgkyOSUwt4AXv58gvUgAPxiElKyCspqGlo5wHoGUEaFOMVWtlAgDPaOzgA0UIpgNFACqgDuAj7GZNvcwK417nUhUFTOXLTNgcEiXRG96QNZ2rljRuZP3qYpiVZjsABYCTY0UHAa4MBJhUEMMCqOJQcEANxhimAoDOQA

Related Issues: n/a

Commencez par la reproduction minimale dans les fonctions x, y et z, ou ouvrez le TypeScript Playground lié. Comparez les diagnostics pour les propriétés requises, explicitement définies sur undefined et optionnelles. C’est terminé lorsque la comparaison dans z signale TS2367 comme x et y, tout en préservant le comportement existant pour les autres exemples.