Assertion signature on generics doesn't narrow

🔎 Search Terms

Assertion AND signature

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Assertion Signatures, Generics.

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&allowUnusedLabels=true&downlevelIteration=true&noEmitHelpers=true&noUnusedLocals=true&noUnusedParameters=true&preserveConstEnums=true&removeComments=true&importHelpers=true&target=99&useUnknownInCatchVariables=false&exactOptionalPropertyTypes=true&noImplicitOverride=true&noFallthroughCasesInSwitch=true&noPropertyAccessFromIndexSignature=true&inlineSourceMap=true&inlineSources=true&stripInternal=true&ts=5.6.2#code/FAehAICIHEHtYCbgIYGdUFMBOAXSAucAW2QGsNVwEKcsBXAYxzqwEsA7Ac3FUYYwwJQEAGZ12TVrHYp02HAAoGhcaXawA7uwCUhNJlyUG4AN7DwFi6xHgFAQgbbT5y6-A4AFlk3h2GDeAAKgCeAA4YAKJY3lgKAOQAgnK4UjIiyKwANoJx2gDcLpYAvuYl5pAAQshI+vIEVDT0TCwc3NgxwGISOKmyBjgAPADCAHxKhEO6ffJGzq7Wtg5OZm6Wnt4BfgEh4VEx8Un9velZOfnAriVlGAAeobC44F2S0u40CpkcFISotK0A2gBdKa-NhcIHOC6rcDZHBPcTKObQ1zsZBEDA-P5cArI1yfPyoTFgzhAnHQorgAA+vjomUy4AAvDS6QUoasRA9bAxpL8YV9wLAbPiKMs2biFgoALLITwAOiwyHYCFgRAUTgG4AADLKAKyi3HQ56MpEG5Go9GESAARw02EgABoxaa3MLCeB-sLAY7nZcyQaKRhMpgTT7argFM9zj7LNz2LyTOaMPa+QSKUznnlLGBwAMGXnwBEAEqFgDyhfAAAkixEnabXbLQnRUB4Pl8o9HpuHE+A7Pm4sgAEYMXJ+3ElZHj6ESyO11ZYDDMLBpBGy12j1zzxcyNdiopAA

💻 Code

// "Good assert": makes destructuring succeed
// function assert(c: unknown): asserts c {
//     if (!c) {
//         throw new TypeError('Assertion failed');
//     }
// }

// "Bad assert": destructuring error
function assert<C>(c: C): asserts c {
    if (!c) {
        throw new TypeError('Assertion failed');
    }
}

export function test(lines: string[]): string[] {

        let func: {
            name: string;
            lines: string[];
        } | null = null;

        for (const line of lines) {
            if (Math.random() < 0.5) {
                func = {
                    name: "qwer",
                    lines: [line],
                };
            } else {
                assert(func);
                const {name, lines} = func;   // <=== ERROR HERE: 'name' implicitly has type 'any'
                lines.push(line);
                assert(name !== 'abc');
            }
        }
        if (func)
            return func.lines;
        return lines;
    }

🙁 Actual behavior

When the generic-assert implementation is active, the line -

const {name, lines} = func;

errors with 'name' implicitly has type 'any'.

When the unknown assert implementation is active, types are properly inferred.

🙂 Expected behavior

Both assert versions contain assertion signature, and both should be able to narrow the type.

Additional information about the issue

No response