Tslint: unknown and strict-type-predicate

[ThomasdenH]

Bug Report

• __TSLint version__: 5.11.0
• __TypeScript version__: 3.0.1
• __Running TSLint via__: CLI

TypeScript code being linted

const body: unknown = 'test';
if (typeof body === 'object') // Expression is always true.
  console.log('a');

with tslint.yaml configuration:

---
rules:
  strict-type-predicates: true

Actual behavior

tslint falsely warns that the expression is always true.

Expected behavior

Since the object is of type unknown (and actually a string), the expression could be both true and false and thus tslint should not warn.

[InExtremaRes]

Same here. In fact unknown should be treated the same as any with regards to this rule.

[LehaIvanov]

Same problem when comparing unknown type with undefined. As says TypeScript documentation, anything (including undefined) is assignable to unknown type, so comparing it with undefined is always correct.

class TestClass {
    private readonly myVar: unknown = "Test";
    public constructor() {
        this.myVar = undefined;
        if (this.myVar !== undefined) { // Expression is always true.
        }
    }
}