Pattern match variable scope

Question

In the Roslyn Pattern Matching spec it states that:

The scope of a pattern variable is as follows:

If the pattern appears in the condition of an if statement, its scope is the condition and controlled statement of the if statement, but not its else clause.

However the latest Microsoft "What's new" posts and presentations are showing this example:

public void PrintStars(object o)
{
    if (o is null) return;     // constant pattern "null"
    if (!(o is int i)) return; // type pattern "int i"
    WriteLine(new string('*', i));
}

Which shows the pattern match i variable used outside the if level scope of the pattern match.

Is this an oversight, or has the scoping been changed from the spec?

Answer 1

From that same documentation:

the variables introduced by a pattern – are similar to the out variables described earlier

So actually this code:

if (!(o is int i)) return; // type pattern "int i"

Is more or less equal to:

int i;
if (!(SomeParsingOn(o, out i))) return; // type pattern "int i"

That means that i is declared on the same level as the if, which means it is in scope not only for the if, but also for following statements. That this is true can be seens when you copy the if:

if (!(o is int i)) return; // type pattern "int i"
if (!(o is int i)) return; // type pattern "int i"

Gives error CS0128: A local variable named 'i' is already defined in this scope.