TypeScript Version:

• 3.4.0-dev.20190130
• 3.2.4

Search Terms:

• ts8020
• "jsdoc types can only be used"
• "JSDoc types can only be used inside documentation comments"

Code

function isString(str: string): str is string! {
  return typeof str === 'string';
}

Expected behavior:

Error message should not mention JSDoc syntax. IMHO it should be TS1144: '{' or ';' expected., which is the case with this code for example:

function isString(str: string): str is string? {
  return typeof str === 'string';
}

Actual behavior:

Error message states that this is JSDoc syntax (am not sure it even is). Full output of tsc:

> tsc test.ts
test.ts:1:40 - error TS8020: JSDoc types can only be used inside documentation comments.

1 function isString(str: string): str is string! {
                                         ~~~~~~~


Found 1 error.

Playground Link:
here

Related Issues:

• PR Quickfix jsdoc in Typescript files #17250