C++: Bring back the StackVariable QL class

The new `StackVariable` class actually denotes what its name suggests.

Author: jbj

cpp/ql/src/semmle/code/cpp/Variable.qll

@@ -307,21 +307,30 @@ class ParameterDeclarationEntry extends VariableDeclarationEntry {
  * }
  * ```
  *
- * Local variables can be static; use the `isStatic` member predicate to
- * detect those.
+ * See also `StackVariable`, which is the class of local-scope variables
+ * without statics and thread-locals.
  */
 class LocalScopeVariable extends Variable, @localscopevariable {
   /** Gets the function to which this variable belongs. */
   /*abstract*/ Function getFunction() { none() }
 }
 
 /**
- * DEPRECATED: use `LocalScopeVariable` instead.
+ * A C/C++ variable with _automatic storage duration_. In other words, a
+ * function parameter or a local variable that is not static or thread-local.
+ * For example, the variables `a` and `b` in the following code.
+ * ```
+ * void myFunction(int a) {
+ *   int b;
+ *   static int c;
+ * }
+ * ```
  */
-deprecated class StackVariable extends Variable {
-  StackVariable() { this instanceof LocalScopeVariable }
-
-  Function getFunction() { result = this.(LocalScopeVariable).getFunction() }
+class StackVariable extends LocalScopeVariable {
+  StackVariable() {
+    not this.isStatic() and
+    not this.isThreadLocal()
+  }
 }
 
 /**
@@ -496,7 +505,7 @@ class TemplateVariable extends Variable {
  * `myTemplateFunction<T>`:
  * ```
  * void myFunction() {
- *   T a;
+ *   float a;
  * }
  *
  * template<type T>
@@ -509,9 +518,6 @@ class TemplateVariable extends Variable {
  * myTemplateFunction<int>();
  * ```
  */
-class SemanticStackVariable extends LocalScopeVariable {
-  SemanticStackVariable() {
-    not this.isStatic() and
-    not this.isFromUninstantiatedTemplate(_)
-  }
+class SemanticStackVariable extends StackVariable {
+  SemanticStackVariable() { not this.isFromUninstantiatedTemplate(_) }
 }

cpp/ql/test/library-tests/variables/variables/variable.expected

@@ -2,13 +2,9 @@
 | file://:0:0:0:0 | gp_offset | file://:0:0:0:0 | unsigned int | Field |  |  |
 | file://:0:0:0:0 | overflow_arg_area | file://:0:0:0:0 | void * | Field |  |  |
 | file://:0:0:0:0 | p#0 | file://:0:0:0:0 | __va_list_tag && | SemanticStackVariable |  |  |
-| file://:0:0:0:0 | p#0 | file://:0:0:0:0 | __va_list_tag && | StackVariable |  |  |
 | file://:0:0:0:0 | p#0 | file://:0:0:0:0 | address && | SemanticStackVariable |  |  |
-| file://:0:0:0:0 | p#0 | file://:0:0:0:0 | address && | StackVariable |  |  |
 | file://:0:0:0:0 | p#0 | file://:0:0:0:0 | const __va_list_tag & | SemanticStackVariable |  |  |
-| file://:0:0:0:0 | p#0 | file://:0:0:0:0 | const __va_list_tag & | StackVariable |  |  |
 | file://:0:0:0:0 | p#0 | file://:0:0:0:0 | const address & | SemanticStackVariable |  |  |
-| file://:0:0:0:0 | p#0 | file://:0:0:0:0 | const address & | StackVariable |  |  |
 | file://:0:0:0:0 | reg_save_area | file://:0:0:0:0 | void * | Field |  |  |
 | variables.cpp:1:12:1:12 | i | file://:0:0:0:0 | int | GlobalVariable |  |  |
 | variables.cpp:2:12:2:12 | i | file://:0:0:0:0 | int | GlobalVariable |  |  |
@@ -38,11 +34,10 @@
 | variables.cpp:37:6:37:8 | ap3 | file://:0:0:0:0 | int * | GlobalVariable |  |  |
 | variables.cpp:41:7:41:11 | local | file://:0:0:0:0 | char[] | LocalVariable |  |  |
 | variables.cpp:41:7:41:11 | local | file://:0:0:0:0 | char[] | SemanticStackVariable |  |  |
-| variables.cpp:41:7:41:11 | local | file://:0:0:0:0 | char[] | StackVariable |  |  |
 | variables.cpp:43:14:43:18 | local | file://:0:0:0:0 | int | LocalVariable |  | static |
-| variables.cpp:43:14:43:18 | local | file://:0:0:0:0 | int | StackVariable |  | static |
 | variables.cpp:48:9:48:12 | name | file://:0:0:0:0 | char * | Field |  |  |
 | variables.cpp:49:12:49:17 | number | file://:0:0:0:0 | long | Field |  |  |
 | variables.cpp:50:9:50:14 | street | file://:0:0:0:0 | char * | Field |  |  |
 | variables.cpp:51:9:51:12 | town | file://:0:0:0:0 | char * | Field |  |  |
 | variables.cpp:52:16:52:22 | country | file://:0:0:0:0 | char * | MemberVariable |  | static |
+| variables.cpp:56:14:56:29 | externInFunction | file://:0:0:0:0 | int | GlobalVariable |  |  |

cpp/ql/test/library-tests/variables/variables/variables.cpp

@@ -51,3 +51,7 @@ struct address {
   char* town;
   static char* country;
 };
+
+void hasExtern() {
+  extern int externInFunction;
+}