Nullable array types are accepted in declaration patterns

[alrz]

Version Used: NullableReferenceTypes (2f8fefa)

Steps to Reproduce:

    public void M(object o) {
        _ = o is string[]?;
        _ = o is string?;
        _ = o is string[]? a; // no error (incorrect)
        _ = o is string? b; // error
    }

Expected Behavior:

Error on third line as well. Patterns don't support nullable types.

Plus, I don't know why the first two are decided to be valid.

Actual Behavior:

Errors only on non-array nullables in patterns.

[alrz]

Also I think there's room for diagnostic clarity instead of giving syntax errors.

[jcouv]

FYI @gafter

[gafter]

if (o is A[]   ? b
                   : c)  // OK, this means `(o is A[]) ? b : c` because nullable types are not permitted for a pattern's type

if (o is A[]   ? b
                   && c) // error: 'cannot use nullable reference type for a pattern' or 'expected :'

if (o is A[][] ? b
                   : c)  // OK, this means `(o is A[][]) ? b : c`

if (o is A[][] ? b
                   && c) // OK, this means `(o is A[][]? b) && c` because `A[][]?` is not a nullable type

[alrz]

Another case where this is incorrectly accepted:

#nullable enable
public class C {
    public void M() {
    	_ = new C?(); // error 
        _ = new string[50]?; // no error (incorrect)
        _ = new string[50]?[,]; // no error (incorrect)
    }
}

Looks like the same issue.

[gafter]

Fixed in #32432