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

Reproduce the narrowing failure from the linked TypeScript Playground, comparing the generic assertion with the unknown-parameter assertion. Trace assertion-signature handling and generic control-flow narrowing in the compiler, then add a regression test showing that destructuring after assert(func) infers name and lines without an implicit-any error.