Yielding an assertion function call does not narrow the asserted type

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

💻 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;

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.

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.

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

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Yielding an assertion function call does not narrow the asserted type #55205

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🙁 Actual behavior

🙂 Expected behavior

Workaround

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Yielding an assertion function call does not narrow the asserted type #55205

Description

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Workaround

Activity

MartinJohns commented on Jul 30, 2023

RyanCavanaugh commented on Aug 1, 2023

typescript-bot commented on Aug 4, 2023

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