Missing warning: the given expression is never of the provided type

[gafter]

In this code

class C<T>
{
    void M(int x)
    {
        if (x is C<T>) { } // warning: the given expression is never of the provided ('C<T>') type.
    }
    void M(C<T> x)
    {
        if (x is int) { } // MISSING warning: the given expression is never of the provided ('int') type.
    }
}

We give a warning where indicated, but are missing the warning on the second case.

[gafter]

The code to detect this (missing) warning is common with the code to detect this case for pattern-matching

    void M(C<T> x)
    {
        if (x is 3) { } // error: input of type 'C<T>' can never match a pattern of type 'int'.
    }

[gafter]

Fixed in #25995