Roslyn: "Use pattern matching" code fix provider produces invalid code

[bartdesmet]

Version Used: VS 16.0 Preview 1.1

Steps to Reproduce:

static bool F(object o1, object o2)
{
    var s1 = o1 as string;
    var s2 = o2 as string;

    if (s1 != null && s2 != null)
    {
        return s1 == s2;
    }

    if (s1 == null || s2 == null)
    {
        return false;
    }

    return o1 == o2;
}

Actual Behavior:

Applying suggested "Use pattern matching" code fixes produce the following code:

static bool F(object o1, object o2)
{
    if (s1 != null && s2 != null)
    {
        return s1 == s2;
    }

    if (!(o1 is string s1) || !(o2 is string s2))
    {
        return false;
    }

    return o1 == o2;
}

The declaration site of s1 and s2 is beyond the first use of the variables, thus making the first line produce "use before declare" errors.

[bartdesmet]

Simpler repro:

static void F(object o)
{
    var s = o as string;

    if (s != null)
    {
        return;
    }

    if (s == null)
    {
        return;
    }
}

produces

static void F(object o)
{
    if (s != null)
    {
        return;
    }

    if (!(o is string s))
    {
        return;
    }
}

[alrz]

I think this is the same issue that https://github.com/dotnet/roslyn/pull/32440 fixed. (use of pattern variable before null check).

[jinujoseph]

Dave , can you validate this with latest build

[dibarbet]

Verified that there is no longer a pattern matching suggestion after https://github.com/dotnet/roslyn/pull/32440. This fix will go into 16.0.Preview3