Inconsistent unreachable code detection

[leetwinski]

Bug Report

Unreachable code detection seems to be inconsistent in case of assigning the never returning function call result to a var.
In this case the unreachable code error isn't signaled, while it gets signaled then there's no such assignment.

🔎 Search Terms

unreachable code

🕗 Version & Regression Information

4.7.4

⏯ Playground Link

Playground link with relevant code

💻 Code

function f(): never {
    f()
}


function f1(x: number): number {
    f();
    return x;  // Unreachable code error
}

function f2(x: number): number {
    const zee = f();
    return x;  // Successful compilation
}

🙁 Actual behavior

the last line of f2 function doesn't fire an unreachable code compilation error

🙂 Expected behavior

the same as that of f1

[fatcerberus]

This is a known limitation. See #32695; in particular:

A function call is analyzed as an assertion call or never-returning call when

• the call occurs as a top-level expression statement, and
• the call specifies a single identifier or a dotted sequence of identifiers for the function name, and
• each identifier in the function name references an entity with an explicit type, and
• the function name resolves to a function type with an asserts return type or an explicit never return type annotation.

[leetwinski]

@fatcerberus, thank you for your answer!
Wouldn't it be nice to have at least warning about never on the right side in assignment?

[fatcerberus]

never is intentionally assignable to every other type - somewhat nerdy theoretical explanation here:
#49862 (comment)

Also see extensive discussion about assignability from never in #28982.

[leetwinski]

#49862 (comment)

Wow that makes it clear from the set theory's point of view. Nevertheless it is still rather confusing for the working programmer )
Well, i guess i'll have to live with that
Once again, thanks for the explanation!

[fatcerberus]

@leetwinski Also, from a type theoretical point of view, a function that normally returns a number but might throw could be seen as returning number | never. Generally you want the result of this function to be assignable to number, and if never is assignable to everything, the union can be treated as just number, making it so.