Bug Report

Invalid type narrowing seems to bleed out of expressions that includes "useless" checks, such as :

• checking the existence of a property marked as required
• using optional chaining when the variable is never null

🔎 Search Terms

• invalid type narrowing
• extra condition narrowing

🕗 Version & Regression Information

Tested in 4.4.4, 4.5.4 and 4.6.0-dev.20220107
This seems to be the behavior in every version I tried, and I didn't find any relevant point if the FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = { a: string };
type B = { a: string; b: string };

// With useless `'a' in x`
declare const value1: A | B;
'a' in value1 && 'b' in value1 && value1.b;
'a' in value1 && 'b' in value1 && value1.b; // Error

// With useless optionnal chaining
declare const value2: A | B;
'b' in value2 && value2?.b;
'b' in value2 && value2.b; // Error

🙁 Actual behavior

In both case, we get the error Property 'b' does not exist on type 'never'.(2339)
The variable is narrowed to A after the first expression, which cause the next one to fail.

🙂 Expected behavior

No type narrowing should occur outside of the expressions.