Merge pull request #3397 from bravequickcleverfibreyarn/ch18-03-pattern-syntax.md

ch18-03: Guarded match arm exhaustivness clarification

src/ch18-03-pattern-syntax.md

@@ -477,8 +477,27 @@ second arm doesn’t have a match guard and therefore matches any `Some` variant
 
 There is no way to express the `if x % 2 == 0` condition within a pattern, so
 the match guard gives us the ability to express this logic. The downside of
-this additional expressiveness is that the compiler doesn't try to check for
-exhaustiveness when match guard expressions are involved.
+this additional expressiveness is that the compiler is not smart enough about 
+arms exhaustiveness anymore when match guard expressions are involved.
+
+```rust,ignore,does_not_compile
+match Some(100) {
+    Some(_) if true => print!("Got Some"),
+    None => print!("Got None")
+}
+```
+This `match` expression involves compilation error.
+```console
+error[E0004]: non-exhaustive patterns: `Some(_)` not covered
+   --> src/main.rs:33:11
+    |
+33  |     match Some(100) {
+    |           ^^^^^^^^^ pattern `Some(_)` not covered
+    | 
+```
+Compilation fails even though `Some(_) if true` guarded pattern matches 
+any possible `Some`. Same as unguarded `Some(_)` does.
+
 
 In Listing 18-11, we mentioned that we could use match guards to solve our
 pattern-shadowing problem. Recall that we created a new variable inside the