CPP: Make Handlers always begin a BasicBlock.

cpp/ql/src/semmle/code/cpp/controlflow/BasicBlocks.qll

@@ -240,16 +240,15 @@ class BasicBlock extends ControlFlowNodeBase {
 
   /**
    * Holds if control flow may reach this basic block from a function entry
-   * point or a `catch` clause of a reachable `try` statement.
+   * point or any handler of a reachable `try` statement.
    */
   predicate isReachable() {
     exists(Function f | f.getBlock() = this)
     or
     exists(TryStmt t, BasicBlock tryblock |
-      (
-        this = t.getACatchClause() or
-        this.(Handler).getTryStmt() = t
-      ) and
+      // a `Handler` preceeds the `CatchBlock`, and is always the beginning
+      // of a new `BasicBlock` (see `primitive_basic_block_entry_node`).
+      this.(Handler).getTryStmt() = t and
       tryblock.isReachable() and
       tryblock.contains(t)
     ) or

cpp/ql/src/semmle/code/cpp/controlflow/internal/PrimitiveBasicBlocks.qll

@@ -44,6 +44,14 @@ private cached module Cached {
     // that the node have at least one successor.
     or
     (not successors_extended(_, node) and successors_extended(node, _))
+
+    // An exception handler is always the start of a new basic block. We
+    // don't generate edges for [possible] exceptions, but in practice control
+    // flow could reach the handler from anywhere inside the try block that
+    // could throw an exception of a corresponding type. A `Handler` usually
+    // needs to be considered reachable (see also `BasicBlock.isReachable`).
+    or
+    node instanceof Handler
   }
 
   /** Holds if `n2` follows `n1` in a `PrimitiveBasicBlock`. */

cpp/ql/test/library-tests/c++_exceptions/unreachable.expected

@@ -1,2 +1,2 @@
-| exceptions.cpp:25:13:30:9 | ExprStmt |
+| exceptions.cpp:25:13:25:19 | ExprStmt |
 | exceptions.cpp:26:13:26:13 | ExprStmt |