Control Flow Analysis of Aliased Conditions and Discriminants doesn't work with === true

[andrenanninga]

Bug Report

🔎 Search Terms

• Control flow
• Type narrowing

🕗 Version & Regression Information

This is behavior that exists in every version, expected to work since v4.4 when "Control Flow Analysis of Aliased Conditions and Discriminants" was introduced.

⏯ Playground Link

Playground link with relevant code

💻 Code

function foo(arg: unknown) {
  const argIsString = typeof arg === "string";
  
  if (argIsString) {
    console.log(arg.toUpperCase());
    // No error, works as expected
  }

  if (argIsString === true) {
    console.log(arg.toUpperCase());
    //          ~~~
    // Error! Object is of type 'unknown'.
  }
}

🙁 Actual behavior

Using if (argIsString) correctly narrows arg to type string. Using if (argIsString === true) however does not narrow the arg parameter to string.

🙂 Expected behavior

I expect if (argIsString) & if (argIsString === true) to have the same behavior in terms of type narrowing in this case.

[MartinJohns]

Duplicate of #44366. Last duplicate was created not even a day ago: #49573