ES.84

CppCoreGuidelines.md

@@ -8967,6 +8967,7 @@ Statement rules:
 * [ES.77: Minimize the use of 'break' and `continue` in loops](#Res-continue)
 * [ES.78: Always end a non-empty `case` with a `break`](#Res-break)
 * [ES.79: Use `default` to handle common cases (only)](#Res-default)
+* [ES.84: Don't (try to) declare a local variable with no name](#Res-noname)
 * [ES.85: Make empty statements visible](#Res-empty)
 * [ES.86: Avoid modifying loop control variables inside the body of raw for-loops](#Res-loop-counter)
 
@@ -10541,6 +10542,35 @@ Flag `switch`-statements over an enumeration that don't handle all enumerators a
 This may yield too many false positives in some code bases; if so, flag only `switch`es that handle most but not all cases
 (that was the strategy of the very first C++ compiler).
 
+### <a name="Res-noname"></a>ES.84: Don't (try to) declare a local variable with no name
+
+##### Reason
+
+There is no such thing.
+What looks to a human like a variable without a name is to the compiler a statement consisting of a temporary that immediately goes out of scope.
+To avoid unpleasent surprises.
+
+###### Example, bad
+
+    void f()
+    {
+        lock<mutex>{mx};   // Bad
+        // ...
+    }
+
+This declares an unnamed `lock` object that immediately goes out of scope at the point of the semicolon.
+This is not an uncommon mistake.
+In particular, this particular example can lead to hard-to find race conditions.
+There are exceedingly clever used of this "idiom", but they are far rarer than the mistakes.
+
+##### Note
+
+    Unnamed function arguments are fine.
+
+##### Enforcement
+
+    Flag statements that are just a temporary
+
 ### <a name="Res-empty"></a>ES.85: Make empty statements visible
 
 ##### Reason