C++: Fix ConditionDeclExpr data flow

Data flow probably never worked when a variable declared in a
`ConditionDeclExpr` was modeled with `BlockVar`. That combination did
not come up in testing before the last commit.

Author: jbj

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -244,7 +244,7 @@ module FlowVar_internal {
       not v instanceof Field and // Fields are interprocedural data flow, not local
       reachable(sbb) and
       (
-        initializer(sbb.getANode(), v, _)
+        initializer(v, sbb.getANode())
         or
         assignmentLikeOperation(sbb, v, _, _)
         or
@@ -361,7 +361,12 @@ module FlowVar_internal {
       assignmentLikeOperation(node, v, _, e) and
       node = sbb
       or
-      initializer(node, v, e) and
+      // We pick the defining `ControlFlowNode` of an `Initializer` to be its
+      // expression rather than the `Initializer` itself. That's because the
+      // `Initializer` of a `ConditionDeclExpr` is for historical reasons not
+      // part of the CFG and therefore ends up in the wrong basic block.
+      initializer(v, e) and
+      node = e and
       node = sbb.getANode()
     }
 
@@ -719,13 +724,11 @@ module FlowVar_internal {
   }
 
   /**
-   * Holds if `v` is initialized by `init` to have value `assignedExpr`.
+   * Holds if `v` is initialized to have value `assignedExpr`.
    */
-  predicate initializer(
-      Initializer init, LocalVariable v, Expr assignedExpr)
+  predicate initializer(LocalVariable v, Expr assignedExpr)
   {
-    v = init.getDeclaration() and
-    assignedExpr = init.getExpr()
+    assignedExpr = v.getInitializer().getExpr()
   }
 
   /**

cpp/ql/test/library-tests/dataflow/fields/flow.expected

@@ -16,6 +16,14 @@ edges
 | A.cpp:73:10:73:19 | call to setOnBWrap [c, ... (1)] | A.cpp:75:10:75:11 | b2 [c, ... (1)] |
 | A.cpp:73:25:73:32 | new [void] | A.cpp:73:10:73:19 | call to setOnBWrap [c, ... (1)] |
 | A.cpp:75:10:75:11 | b2 [c, ... (1)] | A.cpp:75:14:75:14 | c |
+| A.cpp:98:12:98:18 | new [void] | A.cpp:100:5:100:13 | ... = ... [void] |
+| A.cpp:100:5:100:6 | c1 [post update] [a, ... (1)] | A.cpp:101:8:101:9 | c1 [a, ... (1)] |
+| A.cpp:100:5:100:13 | ... = ... [void] | A.cpp:100:5:100:6 | c1 [post update] [a, ... (1)] |
+| A.cpp:101:8:101:9 | c1 [a, ... (1)] | A.cpp:103:14:103:14 | c [a, ... (1)] |
+| A.cpp:103:14:103:14 | c [a, ... (1)] | A.cpp:107:12:107:13 | c1 [a, ... (1)] |
+| A.cpp:103:14:103:14 | c [a, ... (1)] | A.cpp:120:12:120:13 | c1 [a, ... (1)] |
+| A.cpp:107:12:107:13 | c1 [a, ... (1)] | A.cpp:107:16:107:16 | a |
+| A.cpp:120:12:120:13 | c1 [a, ... (1)] | A.cpp:120:16:120:16 | a |
 | A.cpp:126:5:126:5 | b [post update] [c, ... (1)] | A.cpp:131:8:131:8 | ref arg b [c, ... (1)] |
 | A.cpp:126:12:126:18 | new [void] | A.cpp:126:5:126:5 | b [post update] [c, ... (1)] |
 | A.cpp:131:8:131:8 | ref arg b [c, ... (1)] | A.cpp:132:10:132:10 | b [c, ... (1)] |
@@ -169,6 +177,8 @@ edges
 | A.cpp:57:28:57:30 | call to get | A.cpp:57:17:57:23 | new [void] | A.cpp:57:28:57:30 | call to get | call to get flows from $@ | A.cpp:57:17:57:23 | new [void] | new [void] |
 | A.cpp:66:14:66:14 | c | A.cpp:64:21:64:28 | new [void] | A.cpp:66:14:66:14 | c | c flows from $@ | A.cpp:64:21:64:28 | new [void] | new [void] |
 | A.cpp:75:14:75:14 | c | A.cpp:73:25:73:32 | new [void] | A.cpp:75:14:75:14 | c | c flows from $@ | A.cpp:73:25:73:32 | new [void] | new [void] |
+| A.cpp:107:16:107:16 | a | A.cpp:98:12:98:18 | new [void] | A.cpp:107:16:107:16 | a | a flows from $@ | A.cpp:98:12:98:18 | new [void] | new [void] |
+| A.cpp:120:16:120:16 | a | A.cpp:98:12:98:18 | new [void] | A.cpp:120:16:120:16 | a | a flows from $@ | A.cpp:98:12:98:18 | new [void] | new [void] |
 | A.cpp:132:13:132:13 | c | A.cpp:126:12:126:18 | new [void] | A.cpp:132:13:132:13 | c | c flows from $@ | A.cpp:126:12:126:18 | new [void] | new [void] |
 | A.cpp:152:13:152:13 | b | A.cpp:143:25:143:31 | new [void] | A.cpp:152:13:152:13 | b | b flows from $@ | A.cpp:143:25:143:31 | new [void] | new [void] |
 | A.cpp:152:13:152:13 | b | A.cpp:150:12:150:18 | new [void] | A.cpp:152:13:152:13 | b | b flows from $@ | A.cpp:150:12:150:18 | new [void] | new [void] |