Error property-will-be-overwritten-by-spread for non-enumerable properties

[benurb]

TypeScript Version: 3.9.2, 4.0.0-dev.20200514, latest master (7fc456f)

Search Terms:

• This spread always overwrites this property
• Error spread
• strictNullChecks

Code

const err = new Error("Error reason");
console.log({
    message: err.message,
    ...err
});

Expected behavior:
Compiles sucessfully using strictNullChecks compiler option. Works correctly with typescript 3.8.3.

Actual behavior:
Compilation fails even though the spread does not overwrite anything since message is a non-enumarable property on Error. Without setting strictNullChecks it works correctly in typescript 3.9.2.

test.ts:3:5 - error TS2783: 'message' is specified more than once, so this usage will be overwritten.

3     message: err.message,
      ~~~~~~~~~~~~~~~~~~~~

  test.ts:4:5
    4     ...err
          ~~~~~~
    This spread always overwrites this property.

Playground Link:

Related Issues: #38577 seems to be a fix for a similar problem

[RyanCavanaugh]

TS doesn't track enumerability, so has no way to know that this isn't a misordered spread. See #9726