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

Start with the minimal reproduction in functions x, y, and z, or open the linked TypeScript Playground. Compare the diagnostics for required, explicitly undefined, and optional properties. Done means the comparison in z reports TS2367 like x and y, with existing behavior preserved for the other examples.