Yielding an assertion function call does not narrow the asserted type

[WilcoKruijer]

Bug Report

🔎 Search Terms

generator, yield, assertion function

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about assertion functions and generators.

⏯ Playground Link

Playground link with relevant code

💻 Code

 function * testFunc() {
    type SomeType = {a: number};

    let count = 0;
    function assertAThing(obj: unknown): asserts obj is SomeType {
        // don't care about the impl
        count++;
    }

    const bla: unknown = {a: 3};
    assertAThing(bla);

    // no type error here
    console.log(bla.a);

    const bla2: unknown = {a: 4};
    yield assertAThing(bla2);

    // bla2 is unknown, but should be SomeType as asserted by the assertAThing call.
    if (bla2.a !== 4) {
        throw new Error("bla2.a should be 4.");
    }

    if (count !== 2) {
        throw new Error("Count should be two.");
    }

    console.log("Javascript success")
 }

for (const item of testFunc()) {
    //
}

🙁 Actual behavior

The variable bla2 is still unknown even after passing it through the assertion function assertAThing.

🙂 Expected behavior

bla2 should be narrowed to be SomeType like bla1.

Workaround

We are effectively yielding void since assertion functions cannot return anything. This means there is a simple workaround:

assertAThing(bla2);
yield;

[MartinJohns]

This is not a bug as per #32695:

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.

[RyanCavanaugh]

This doesn't seem like something you would likely (ever?) actually do in real code, and making new CFA graph nodes slows down everything else.

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.