[doc] Documentation for Optional Empty Access

Reviewed By: jvillard

Differential Revision: D24729144

fbshipit-source-id: 9995f68a7

infer/documentation/issues/OPTIONAL_EMPTY_ACCESS.md

@@ -0,0 +1,51 @@
+Optional Empty Access warnings are reported when we try to retrieve the value of a [`folly::Optional`](https://github.com/facebook/folly/blob/master/folly/Optional.h) when it is empty (i.e. `folly::none`).
+
+In the following example we get a warning as `int_opt` might be `folly::none` and its value is being accessed:
+
+```cpp
+bool somef(int v);
+
+folly::Optional<int> mightReturnNone(int v) {
+   if (somef(v)) {
+      return folly::Optional(v);
+   }
+
+   return folly::none;
+}
+
+int value_no_check() {
+  folly::Optional<int> int_opt = mightReturnNone (4);
+  return int_opt.value(); // Optional Empty Access warning
+}
+```
+
+We do not get the warning anymore if we add a check whether `int_opt` is not empty:
+
+```cpp
+int value_check() {
+  folly::Optional<int> int_opt = mightReturnNone (4);
+  if (int_opt.has_value()) {
+     return int_opt.value(); // OK
+  }
+  return -1;
+}
+```
+
+In some cases we know that we have a non-empty value and there is no need to have a check. Consider the following example where Infer does not warn:
+
+```cpp
+bool somef(int v) {return v > 3;};
+
+folly::Optional<int> mightReturnNone(int v) {
+   if (somef(v)) {
+      return folly::Optional(v);
+   }
+
+   return folly::none;
+}
+
+int value_no_check() {
+  folly::Optional<int> int_opt = mightReturnNone (4); // cannot be folly::none
+  return int_opt.value(); // OK
+}
+```

infer/src/base/IssueType.ml

@@ -764,7 +764,7 @@ let nullptr_dereference =
 
 let optional_empty_access =
   register ~enabled:false ~id:"OPTIONAL_EMPTY_ACCESS" Error Pulse
-    ~user_documentation:"Reports on accessing folly::Optional when it is none."
+    ~user_documentation:[%blob "../../documentation/issues/OPTIONAL_EMPTY_ACCESS.md"]
 
 
 let parameter_not_null_checked =