New C#7 pattern matching and reachability

Question

This is based on this question, which seems to me a language glitch or a missing warning in the compiler. My reasoning is that is will always return true in expressions of the following type:

int i;
if (i is var j) ....

Taking it a step further, consider the following code:

int i;
if (i is int) .... //Compiler warning: The given expression is always
                   //of the provided ('int') type.

But, same as with var, if you do:

if (i is int j) ...
else ...

You won't get any warning (neither given expression is always true or unreachable code detected in the else clause) to what, essentially, is the same behavior.

This makes me consider if maybe my initial reasoning is wrong. Is there a some obscure use/corner case I'm missing where this really makes a difference? And if not, is the missing warning and the inconsistent behavior an oversight (or even bug) in the compiler?

Answer 1

I believe this is by design, referencing the roslyn source code, starting here: Binder_Operators, Line 2848

Because the heuristic presented here is used to change codegen, it must be conservative. It is acceptable for us to fail to report a warning in cases where humans could logically deduce that the operator will always return [true]. It is not acceptable to inaccurately warn that the operator will always return [true] if there are cases where it might [fail].