Pattern Matching：is var XX and，XX can be used by other paths

[vst-h]

Expect is var XX and to be like is var XX &&, making variable XX available in other paths.
Code example:

    public Task<List<int>> GetListA() => Task.FromResult(new List<int>());

    public Task<(int, int, int)> GetTupleA() => Task.FromResult((1, 2, 3));

    public async Task Test() {
        if (await GetListA() is var list1 && list1.Count == 0 && list1 is []) {
            throw new Exception();
        }
        _ = list1; // can use

        if (await GetListA() is var list2 and { Count: 0 } and []) {
            throw new Exception();
        }
        _ = list2; // error: Use of unassigned local variable 'list2'

        // There are other types of uses

        if (await GetTupleA() is var (a1, b1, c1) and (>10, >100, >1000)) {
            throw new Exception();
        }
        _ = (a1, b1, c1); // error

        if (await GetTupleA() is var (a2, b2, c2) && a2 >10 &&  b2 >100 && c2 >1000) {
            throw new Exception();
        }
        _ = (a2, b2, c2); // can use
    }

Language version of the above code：

    <TargetFramework>net7.0</TargetFramework>
    <LangVersion>preview</LangVersion>

[CyrusNajmabadi]

I don't understand what this discussion is asking. Closing out given no actual question provided.

[vst-h]

Sorry for the disturbance, it's been updated and added.

[HaloFour]

I think it has to do with the way that pattern combinators behave differently from Boolean operators in conditional statements, specifically in how they impact definite assignment. But I don't know what the discussion is other than pointing it out.

[vst-h]

The example code expects the variable list2 to be available after an if statement, and similarly the variables a1, b1, c1.

[HaloFour]

The variable list2 is available, but it is not definitely assigned. If any part of the pattern fails, the entire pattern fails, thus the pattern variable never receives a value. That's what makes it different from the comparable logic using Boolean operators, and that is an intentional part of the design.